Sidney H. Reynolds

THE EXOSKELETON AND VERTEBRAL COLUMN.

Epidermal Exoskeleton.

Hair, which forms the characteristic Mammalian exoskeleton, varies much in different animals, and in different parts of the same animal. A large proportion of mammals have the surface fairly uniformly covered with hair of one kind only. In some forms however there are two kinds of hair, a longer and stiffer kind alone appearing on the surface, and a shorter and softer kind forming the under fur. In most mammals hairs of a special character occur in certain regions, such as above the eyes, on the margins of the eyelids, and on the lips and cheeks, here forming the vibrissae or whiskers.

Sometimes as in Hippopotamus, Orycteropus and the Sirenia, the hair, though scattered over the whole surface, is extremely scanty, while in the Cetacea it is limited to a few bristles in the neighbourhood of the mouth, or may even be absent altogether in the adult. In most mammals the hairs are shed and renewed at intervals, sometimes twice a year, before and after the winter. The vibrissae or large hairs which occur in many animals upon the upper lip, and the mane and tail of Equidae are probably persistent.

In the hedgehogs, porcupines and Echidna certain of the hairs are modified and greatly enlarged, forming stiff spines. Similar spines occur in the young of Centetes, and in Acanthomys among the Muridae.

Several other forms of epidermal exoskeleton are met with in mammals, including:—

(a) Scales. These overlie the bony scutes of armadillos and occur covering the tail in several groups of mammals, such as beavers and rats. In the Manidae the body is covered by flat scales which overlap.

(b) The horns of Bovine Ruminants. These, which must on no account be confused with antlers, are hollow cases of hardened epidermis fitting on to bony outgrowths of the frontals. In almost every case they are unbranched structures growing continuously throughout life, and are very rarely shed entire. In the Prongbuck Antilocapra however they are bifurcated and are periodically shed. Horns are nearly always limited to a single pair, but the four-horned antelope Tetraceros has two pairs, the anterior pair being the smaller.

(c) The horns of Rhinoceroses. These are conical structures composed of a solid mass of hardened epidermal cells growing from a cluster of long dermal papillae. From each papilla there grows a fibre which resembles a thick hair, and cementing the whole together are cells which grow from the interspaces between the papillae. These fibres differ from true hairs in not being developed in pits in the dermis. Rhinoceros horns may be either one or two in number, and are borne on the fronto-nasal region of the skull. They vary much in length, the longest recorded having the enormous length of fifty-seven inches.

(d) Nails, hoofs and claws. In almost all mammals except the Cetacea, these are found terminating the digits of both limbs. Nails are more or less flattened structures, claws are pointed and somewhat curved. In most mammals the nails tend to surround the ends of the digits much more than they do in man. Sometimes the nail of one digit differs from that of all the others; thus the second digit of the pes in the Hyracoidea and Lemuroidea is terminated by a long claw, the other digits having flat nails. In the Felidae the claws are retractile, the ungual phalanx with claw attached folding back when the animal is at rest into a sheath, above, or by the side of the middle phalanx. In the Sloths and Bats enormously developed claws occur, forming hooks by which the animals suspend themselves. In Notoryctes the third and fourth digits of the manus bear claws of great size; similar claws occur in Chrysochloris, being correlated in each case with fossorial habits. The nail at its maximum development entirely surrounds the terminal phalanx of the digit to which it is attached, and is then called a hoof. Hoofs are specially characteristic of the Ungulata.

(e) Spurs and beaks are structures which are hardly represented among mammals, while so characteristic of birds. They are however both found in the Monotremata. In both Echidna and Ornithorhynchus the male has a peculiar hollow horny spur borne on a sesamoid bone articulated to the tibia. The jaws in Ornithorhynchus are cased in horny beaks similar to those of birds, and are provided with horny pads which act as teeth.

(f) Horny plates of a ridged or roughened character occur upon the anterior portion of the palate, and of the mandibular symphysis in all three genera of recent Sirenia; also upon the toothless anterior portion of the palate in Ruminants.

(g) The baleen of whales also belongs to the epidermal exoskeleton. It consists of a number of flattened horny plates arranged in a double series along the palate. The plates are somewhat triangular in form and have their bases attached to the palate at right angles to its long axis, while their apices hang downwards into the mouth cavity. The outer edge of each plate is hard and smooth, while the inner edge and apex fray out into long fibres which look like hair. At the inner edge of each principal plate are subsidiary smaller plates. The plates are formed of a number of fibres each developed round a dermal papilla in the same way as are the fibres forming the horns of Rhinoceros. Baleen and Rhinoceros horn likewise agree in that the fibres are bound together by less hardened epithelial cells, which readily wear away and allow the harder fibres to fray out. The greatest development of baleen occurs in the Northern Right whale, Balaena mysticetus, in which the plates number three hundred and eighty or more on each side, and reach a length of ten or twelve feet near the middle of the series.

Dermal Exoskeleton.

Mammals show two principal kinds of exoskeletal structures which are entirely or partially dermal in origin, viz. the bony scutes of armadillos, and teeth.

The bony scutes of armadillos are quadrate or polygonal in shape and are in general aggregated together, forming several shields protecting various regions of the body. The head is generally protected by a cephalic shield, the anterior part of the body by a scapular, and the posterior by a pelvic shield. The tail is also generally encased in bony rings, and scutes are irregularly scattered over the surface of the limbs. The mid-body region is protected by a varying number of bands of scutes united by soft skin, so as to allow of movement. Corresponding to each dermal scute is an epidermal plate. In Chlamydophorus the scutes are mainly confined to the posterior region where they form a strong vertically-placed shield which coalesces with the pelvis. The anterior part of the body is mainly covered by horny epidermal plates with very little ossification beneath. In the gigantic extinct Glyptodonts the body is covered with a solid carapace formed by the union of an immense number of plates, and there are no movable rings. The top of the head is defended by a similar plate, the tail is generally encased in an unjointed bony tube, and there is commonly a ventral plastron.

In Phocaena phocaenoides the occurrence of vestigial dermal ossicles has been described, and in Zeuglodon the back was probably protected by dermal plates.

Teeth[145].

Teeth are well developed in the vast majority of mammalia, and are of the greatest morphological and systematic importance, many extinct forms being known only by their teeth. Mammalian teeth differ from those of lower animals in various well-marked respects. (1) They are attached only to the maxillae, premaxillae and mandible, never to the palatines, pterygoids or other bones. (2) They frequently have more than one root. (3) They are always, except in some Odontoceti, placed in distinct sockets. (4) They are hardly ever ankylosed to the bone. (5) They are in most cases markedly heterodont. (6) They are commonly developed in two sets, the milk dentition and permanent dentition.

It sometimes happens that teeth after being formed are reabsorbed without ever cutting the gum. This is the case, for instance, with the upper incisors of Ruminants.

The form of mammalian teeth varies much, some are simple conical structures comparable to those of most reptiles, and these may either have persistent pulps, as in the case of the upper canines of the Walrus and the tusks of Elephants, or may be rooted as in most canine teeth. Some teeth have chisel-shaped edges, and this may be their original form, as in the human incisors, or may, as in those of Rodents, be brought about by the more rapid wearing away of the posterior edge, the anterior edge being hardened by a layer of enamel. Then, again, the crown may, as in the majority of grinding teeth, be more or less flattened. The various terms used in describing some of the forms of the surface of grinding teeth are defined on page 345.

Fig. 81. Skull of a young Indian Rhinoceros (R. unicornis),

SHOWING THE CHANGE OF THE DENTITION × 1/7. (Brit. Mus.)

1. nasal.	mc. milk canine.
2. frontal.	mpm_{1}. milk premolar.
3. parietal.	I_{1}. first incisor.
4. zygomatic process of	c. canine.
squamosal.	pm_{2}, pm_{3}, pm_{4}. 2nd, 3rd and
5. jugal.	4th premolars.
mI_{1}. milk incisor.	m1, m2. first and second molars.

The teeth of the Aard Varks are compound, and differ completely from those of all other mammals (see p. 425).

As a rule, the higher the general organisation of an animal the better are its milk teeth developed, and the more do they form a reproduction on a small scale of the permanent set. This fact is well seen in the Primates, Carnivora and Ungulata. The method of notation by which the dentition of any mammal can be briefly expressed as a formula has been already described. The regular mammalian arrangement of teeth for each side is expressed by the formula

i ³/₃ c ¹/₁ pm ⁴/₄ m ³/₃ × 2; total, 44.

Monotremata. In Echidna teeth are quite absent. In the young Ornithorhynchus[146] functional molar teeth of a multi-tubercular type resembling those of some Mesozoic mammalia are present, but in the adult they disappear, their office being discharged by horny plates.

Marsupialia[147] have a heterodont dentition, which has generally been regarded as almost monophyodont, the only tooth which has an obvious deciduous predecessor being the last premolar. The researches of RÖse[148] and KÜkenthal[149] tend to show that the teeth of Marsupials are developed in the same way as in other mammals, and are diphyodont. In the case of the premolars, teeth which are homologous with the permanent teeth of other mammals begin to develop as lateral outgrowths from the milk teeth, but afterwards become absorbed, so that the teeth which actually persist belong to the milk series. The last premolar, however, does as a rule develop and replace its milk predecessor; sometimes, however, as in Didelphys, it takes its place among the milk molars without replacing one of them.

The types of dentition characteristic of the different groups of placental mammals may mostly be paralleled among the Marsupials. Thus among the polyprotodont forms the Didelphyidae or opossums, and some of the Dasyuridae, such as Sarcophilus and Thylacinus, have a typical carnivorous dentition with small incisors, large canines, and molars with pointed compressed crowns. The dental formula of Thylacinus, is i ⁴/₃ c ¹/₁ pm ³/₃ m ⁴/₄, total 46.

In Myrmecobius five or six molar teeth occur on each side, and the total number of teeth reaches fifty-two or fifty-six. The teeth bear rows of tubercles, and resemble those of the Multituberculate mesozoic Mammals[150], more than do those of any other living form. Calcified teeth have recently been described[151] in Myrmecobius earlier than the functional or milk set. This would relegate the milk teeth of mammals in general to a second series, and the permanent teeth to a third. In Notoryctes the dental formula[152] is given as i ³/₂ c ¹/₁ pm ²/₃ m ⁴/₄, total 40. The canines are small, and the anterior molars have strongly developed cusps, and much resemble those of Chrysochloris (Insectivora).

Among the diprotodont types the Phascolomyidae, or Wombats, have a dentition recalling that of the Rodents. All the teeth grow from persistent pulps, and the incisors have enamel only on the anterior surface as in Rodents. The dental formula is i ¹/₁ c ⁰/₀ pm ¹/₁ m ⁴/₄, total 24. There are indications of a vestigial second pair of incisors.

The Macropodidae, or Kangaroos, have a herbivorous dentition with the formula i ³/₁ c ^(0—1)/₀ pm ²/₂ m ⁴/₄. The incisors are sharp and cutting, and are separated by a long diastema or gap from the molars, which have their crowns marked by ridges or cusps. There are indications of several vestigial incisors.

Coenolestes, a remarkable form recently described from America, belongs to the diprotodont section, and is the only living member of the section known outside the Australian region[153]. An exceptional dentition is seen in the case of the extinct Thylacoleo, in which the functional teeth are reduced to two pairs; one pair of large cutting incisors and one of compressed sharp-edged premolars.

Edentata. Some Edentata, viz. the ant-eaters (Myrmecophagidae) are, as far as is known, absolutely toothless at all stages of their existence; being the only mammals except Echidna in which no tooth germs have been discovered; others, viz. the Manidae, though showing foetal tooth germs, are quite toothless in post-foetal life; others, viz. some of the armadillos, have the largest number of teeth met with in land mammals. The teeth are homodont except in the Aard Varks, and grow from persistent pulps. In the sloths (Bradypodidae) and the Megatheriidae, there are five pairs of teeth in the upper and four in the lower jaw. The teeth of sloths consist of a central axis of vasodentine, surrounded firstly by a thin coating of hard dentine, and secondly by a thick coating of cement.

In no living Edentate have the teeth any enamel; it has, however, been described as occurring in certain early Megatheroid forms from S. America[154], and an enamel organ has also been discovered in an embryo Dasypus[155]. In the Armadillos (Dasypodidae) the number of teeth varies from ⁸/₈ or ⁷/₇ in Tatusia, to upwards of ²⁵/₂₅ in Priodon, which therefore may have upwards of a hundred teeth, the largest number met with in any land mammal. In Tatusia all the teeth except the last are preceded by two-rooted milk teeth. The Aard Varks are diphyodont, and milk teeth are also known in a species of Dasypus, but with these exceptions Edentates are, as far as is known, monophyodont. In Glyptodon the teeth are almost divided into three lobes by two deep grooves on each side.

The Aard Varks (Orycteropodidae) are quite exceptional as regards their teeth, which are cylindrical in shape, and are made up of a number of elongated denticles fused together. Each denticle contains a pulp cavity from which a number of minute tubes radiate outwards. These teeth are diphyodont and somewhat heterodont, eight to ten pairs occur in the upper jaw and eight in the lower, but they are not all in place at one time. The last three teeth in each jaw are not preceded by milk teeth[156].

Sirenia. The teeth of Sirenia show several very distinct types, the least modified being that of the extinct Halitheriidae, which have large incisors in the upper jaw, and five or six pairs of tuberculated grinding teeth in each jaw, the anterior ones being preceded by milk teeth.

In both the living genera the dentition is monophyodont. In Manatus the dentition is i ²/₂ pm and m ¹¹/₁₁. The incisors are vestigial, and disappear before maturity. The grinding teeth have square enamelled crowns marked by transverse tuberculated ridges. They are not all present in the jaw at the same time. In Halicore the upper jaw bears a pair of straight tusklike incisors; in the male these have persistent pulps and project out of the mouth; in the female they soon cease to grow and are never cut. They are separated by a long diastema from the grinding teeth which have tuberculated crowns and are ⁵/₅ or ⁶/₆ in number, but are not all in place at once. Several other pairs of slender teeth occur in the young animal, but are absorbed or fall out before maturity. In Rhytina teeth are altogether absent.

Cetacea.

Archaeoceti. Zeuglodon has the following dentition, i ³/₃ c ¹/₁ pm and m ⁵/₅, total 36. The incisors and canines are simple and conical; the cheek teeth are compressed and have serrated cutting edges like those in some seals.

In the Mystacoceti, or whalebone whales, calcified tooth germs probably belonging to the milk dentition are present in the embryo, but they are never functional, and are altogether absent in the adult. The anterior of these germs are simple, the posterior ones are originally complex, but subsequently split up into simple teeth like those of the anterior part of the jaw. Hence according to KÜkenthal, who described these structures, the Cetacean dentition was originally heterodont.

In the living Odontoceti the dentition is homodont and monophyodont. In some cases traces occur of a replacing dentition which never comes to maturity, and renders it probable that the functional teeth of the Odontoceti are really homologous with the milk teeth of other mammals. Some of the dolphins afford the apparently simplest type of mammalian dentition known. The teeth are all simple, conical, slightly recurved structures, with simple tapering roots and without enamel. The dentition is typically piscivorous, being adapted for seizing active slippery animals such as fish. The prey is then swallowed entire without mastication. Sometimes the teeth are excessively numerous, reaching two hundred or more (fifty to sixty on each side of each jaw) in Pontoporia. This multiplication of teeth is regarded by KÜkenthal as due to the division into three parts of numbers of trilobed teeth similar to those of some seals.

In the Sperm whale, Physeter, the lower jaw bears a series of twenty to twenty-five stout conical recurved teeth, while in the upper jaw the teeth are vestigial and remain imbedded in the gum. An extinct form, Physodon, from the Pliocene of Europe and Patagonia is allied to the Sperm whale, but has teeth in both jaws. In the Killer Orca, the teeth number about ¹²/₁₂, and are very large and strong. In some forms the teeth are very much reduced in number; thus in Mesoplodon the dentition consists simply of a pair of conical teeth borne in the mandible. In the Narwhal Monodon the dentition is practically reduced to a single pair of teeth, which lie horizontally in the maxillae, and in the female normally remain permanently in the alveoli. In the male the right tooth remains rudimentary, while the left is developed into an enormous cylindrical tusk marked by a spiral groove. Occasionally both teeth develop into tusks, and there is reason for thinking that two-tusked individuals are generally or always female. In the extinct Squalodon the dentition is decidedly heterodont, and the molars have two roots. The dental formula is

i ³/₃ c ¹/₁ pm ⁴/₄ m ⁷/₇, total 60.

It is probable that the homodont condition of modern Odontoceti is not primitive, but due to retrogressive evolution.

Ungulata.

Just as in the Cetacea a piscivorous dentition is most typically developed, so the Ungulata are, as a group, the most characteristic representatives of a herbivorous dentition in its various forms.

Ungulata vera.

Artiodactyla. As regards the living forms, the Artiodactyla can be readily divided into two groups, namely those with bunodont and those with selenodont teeth. It has, however, been shown that selenodont teeth always pass through an embryonic bunodont stage[157]. The bunodont type is best seen in Pigs and Hippopotami and such extinct forms as Hyotherium. In Hippopotamus the dental formula is i ^(2-3)/_(1-3) c ¹/₁ pm ⁴/₄ m ³/₃.

The incisors and canines of Hippopotamus are very large and grow continuously. The genus Sus, which affords a good instance of an omnivorous type of dentition, has the regular unmodified Mammalian dental formula i ³/₃ c ¹/₁ pm ⁴/₄ m ³/₃, total 44. The canines, specially in the male, are large and have persistent pulps, and the upper canines do not have the usual downward direction but pass outwards and upwards. In the Wart Hog, Phacochaerus, they are enormously large, but a still more extraordinary development of teeth is found in Babirussa. In the male Babirussa the canines, which are without enamel, are long, curved and grow continuously. Those of the upper jaw never enter the mouth, but pierce the skin of the face and curve backwards over the forehead. The dental formula of Babirussa is i ²/₃ c ¹/₁ pm ²/₂ m ³/₃, total 34.

The Wart Hog has a very anomalous dentition, for as age advances all the teeth except the canines and last molars show signs of disappearing; both pairs of persisting teeth are however very large.

Various extinct Ungulata such as Anoplotherium have teeth which are intermediate in character between the bunodont and selenodont types. Anoplotherium has the regular mammalian series of forty-four teeth. The crowns of all the teeth are equal in height, and there is no diastema—an arrangement found in no living mammal but man.

We come now to the selenodont Artiodactyla.

The Tylopoda—camels (Camelidae) and Llamas (Aucheniidae) when young have the full number of incisors, but in the adult the two upper middle ones are lost. The molars are typically selenodont and hypsodont[158]. In the Camel the dental formula is i ¹/₃ c ¹/₁ pm ³/₂ m ³/₃, total 34. The upper incisors, canines and first premolars of the Camel are very small teeth, and the first premolar is separated by a long diastema from the others.

The Tragulina or Chevrotains have no upper incisors, while the canines are largely developed, especially in the male.

The Ruminantia or Pecora are very uniform as regards their dentition. The upper incisors are always absent, for though their germs are developed they are reabsorbed without ever becoming visible, and as a rule the upper canines are absent too, while the lower canines are incisiform. The grinding teeth are typically selenodont, and in the lower jaw form a continuous series separated by a wide diastema from the canines. The dental formula is usually

i ⁰/₃ c 0-¹/₁ pm ³/₃ m ³/₃.

The canines are largely developed in the male Muskdeer (Moschus) and in Hydropotes.

Perissodactyla. The premolars and molars have a very similar structure and form a continuous series of large square teeth with complex crowns. The crowns are always constructed on some modification of the bilophodont[159] plan, as is easily seen in the case of the forms with brachydont teeth, but in animals like the Horse, in which the teeth are very hypsodont, this arrangement is hard to trace. All four premolars in the upper jaw are preceded by milk teeth, while in Artiodactyla the first has no milk predecessors.

In the Tapiridae the grinding teeth are brachydont and the lower ones are typically bilophodont. The last two upper molars have the transverse ridges united by an outer longitudinal ridge. The dentition is i ³/₃ c ¹/₁ pm ⁴/₃ m ³/₃, total 42.

In some of the extinct Perissodactyles such as Lophiodon[160], the dentition is brachydont and bilophodont, the grinding teeth in general resembling the posterior upper molars of the Tapir. The same type of brachydont tooth is seen in Palaeotherium but the transverse ridges are crescentic instead of straight, and are separated from one another by shallow valleys without cement. Some of the Palaeotheridae have the regular series of forty-four teeth.

A complete series of forms is known showing how from the simple brachydont teeth of the Palaeotheridae, were derived the complicated hypsodont teeth of the Equidae. The increase in depth of the tooth was accompanied by increase in the depth and complexity of the enamel infoldings, and of the cement filling them.

Both upper and lower grinding teeth of the Equidae are much complicated by enamel infoldings, but their derivation from the bilophodont type can still be recognised. The diastema in front of the premolars is longer in the living Equidae than in their extinct allies. In the adult horse the dental formula is i ³/₃ c ¹/₁ pm ³/₃ m ³/₃, total 40, with often a vestigial first upper premolar (fig. 82, pm 1). The last molar is not more complex than the others, and in the female the canine is quite vestigial. The incisors are large and adapted for cutting and have the enamel curiously folded in forming a deep pit. The milk dentition is di ³/₃ dc ⁰/₀ dpm ³/₃, total 24. The last milk premolar is not more complex than the premolar that succeeds it. The horse affords an excellent instance of a typically herbivorous type of dentition, the cutting incisors, reduced canines and series of large square flat-crowned grinding teeth being most characteristic.

In Rhinoceros the grinding teeth are much like those of Lophiodon, having an outer longitudinal ridge from which two crescentic transverse ridges diverge. The upper premolars are as complex as the molars, and there are no canines; in some species incisors also are absent. The dental formula is

i ^(0—2)/_(0—1) c ⁰/_(0—1) pm ⁴/₄ m ³/₃.

Fig. 82. Palatal aspect of the cranium and mandible of a Donkey (Equus asinus) × ¹/₅. (Camb. Mus.)

1. supra-occipital.	6. glenoid surface.
2. occipital condyle.	7. vomer.
3. basi-occipital.	i 1, i 3. first and third incisors.
4. vacuity representing the	c. canine.
confluent foramen lacerum	pm 1, pm 2. first and second
posterius and foramen	premolars.
lacerum medium.	m 1. first molar.
5. auditory bulla.

Among the Titanotheriidae Palaeosyops[161] has very brachydont teeth whose crowns have been described as buno-selenodont, the inner pair of columns being bunodont, the outer, selenodont. Similar grinding teeth occur in Chalicotherium. Some of the Titanotheriidae have the regular mammalian series of forty-four teeth.

Subungulata.

Toxodontia. Nesodon has the regular dental formula; its grinding teeth are rooted and the upper ones resemble those of Rhinoceros. The second upper and third lower incisors form ever-growing tusks. There is a marked difference between the deciduous and permanent dentition. Astrapotherium likewise has large rooted cheek teeth of a rhinocerotic type, and each jaw bears a pair of permanently growing tusks, those of the lower jaw being the canines. The dental formula is

i ¹/₃ c ⁰/₁ pm ²/₁ m ³/₃, total 28.

In Toxodon the upper incisors and molars are large and curved and all the teeth have persistent pulps. In Typotherium there are no tusks, but the upper incisors are chisel-like, recalling those of Rodents.

The Condylarthra have brachydont, generally bunodont teeth, with the premolars simpler than the molars. They generally have the regular dental formula.

Hyracoidea. The dental formula of Procavia is usually given as i ½ c ⁰/₀ pm ⁴/₄ m ³/₃, total 34; in young individuals however there occur a second pair of upper incisors which early fall out. The upper incisors resemble those of Rodents in being long and curved and growing from persistent pulps. They are however triangular in transverse section, not rectangular, having two antero-lateral faces covered with enamel and a posterior face without enamel. Their terminations are pointed, not chisel-shaped as in Rodents. The lower incisors (fig. 83, i 1) are pectinate or partially divided by vertical fissures, and the grinding teeth are of the rhinocerotic type.

Amblypoda. Two of the best known forms belonging to this extinct group differ much as regards dentition. For while Coryphodon has the regular dental formula, and the canines of both jaws of moderate size, in Uintatherium the dentition is very specialised, there are no upper incisors, and the upper canines form a pair of enormous tusks. The grinding teeth form a continuous series marked by V-shaped ridges and the dental formula is i ⁰/₃ c ¹/₁ pm ³/₃ m ³/₃ total 34.

Proboscidea. The incisors are composed entirely of dentine and have the form of conical tusks projecting greatly from the mouth. In living forms they are confined to the upper jaw, in some species of the extinct Mastodon however they occur in the lower jaw also. In Dinotherium they are probably absent from the upper jaw, but form a pair of downwardly and backwardly-directed tusks growing from the elongated symphysis of the mandible.

The grinding teeth in the various Proboscidea show a very remarkable series of modifications. In Dinotherium they are bilophodont or else are marked by three straight transverse ridges. The dental formula is i ^0?/₁ c ⁰/₀ pm ²/₂ m ³/₃, and the teeth have the normal method of succession. In Mastodon as in Dinotherium the grinding teeth are marked by transverse ridges, but the ridges are subdivided into conical or mammillary cusps, and similar cusps often occur between the ridges. These cusps are covered with very thick enamel and the spaces between them are not filled up with cement. There are six of these grinding teeth for each side of each jaw but only three are in place at once. The first three are milk teeth as they may be succeeded vertically by others.

In the true Elephants the number and depth of the enamel folds is much increased, and the spaces between the folds are filled up with cement. A very complete series of extinct forms is known with teeth intermediate in character between those of Mastodon and those of the Mammoth and living elephants. The dental formula of Elephas is

di ¹/₀ i ¹/₀ c ⁰/₀ dm ^3—4/_3—4 m ³/₃.

Sir W.H. Flower describes[162] the mode of succession of teeth in Elephants as follows: "As regards the mode of succession that of modern Elephants is as before mentioned very peculiar. During the complete lifetime of the animal there are but six molar teeth on each side of each jaw with occasionally a rudimentary one in front, completing the typical number of seven. The last three represent the true molars of ordinary mammals, those in front appear to be milk molars which are never replaced by permanent successors, but the whole series gradually moves forwards in the jaw, and the teeth become worn away and their remnants cast out in front while development of others proceeds behind. The individual teeth are so large and the processes of growth and destruction by wear take place so slowly, that not more than one or portions of two teeth are ever in place and in use on each side of each jaw at one time, and the whole series of changes coincides with the usual duration of the animal's life. On the other hand the Dinotherium, the opposite extreme of the Proboscidean series, has the whole of the molar teeth in place and use at one time, and the milk molars are vertically displaced by premolars in the ordinary fashion. Among Mastodons transitional forms occur in the mode of succession as well as in structure, many species showing a vertical displacement of one or more of the milk molars, and the same has been observed in one extinct species of Elephant (E. planifrons) as regards the posterior of these teeth."

In the Tillodontia the grinding teeth are of Ungulate type, while the second incisors are large and grow from persistent pulps, so as to resemble those of Rodents.

Rodentia have a most characteristic and very constant dentition, the common dental formula being

i ¹/₁ c ⁰/₀ pm ^(0—1)/_(0—1) m ³/₃, total 18 or 20.

The incisors always have chisel-like edges and persistent pulps, and are separated by a wide diastema from the premolars. Canines are always absent, and there are generally three grinding teeth not preceded by milk teeth; their surface may be grooved, or may be bunodont. Teeth are most numerous in the Duplicidentata (Hares and Rabbits), in which the formula is i ²/₁ c ⁰/₀ pm ³/₂ m ³/₃, total 28, and fewest in Hydromys and certain other forms, in which the formula is i ¹/₁ c ⁰/₀ pm ⁰/₀ m ²/₂, total 12. The hares and rabbits are the only rodents which have well developed deciduous incisors, though a vestigial milk incisor has been described in the Mouse (Mus musculus). The last upper molar of Hydrochaerus is very complicated, its structure approaching that of the teeth of Elephants.

In Carnivora vera the incisors are almost always ³/₃. The fourth upper premolar and first lower molar are differentiated as carnassial teeth (see p. 436), and retain fundamentally the same characters throughout the suborder. The upper carnassial (fig. 84, I. II. III.) consists of a more or less compressed, commonly trilobed blade borne on two roots, with an inner tubercle borne on a third root. The lower carnassial has only two roots; its crown consists of a bilobed blade with generally an inner cusp, and a heel or talon (fig. 84, 4) behind the blade.

The most thoroughly carnivorous type of dentition is seen in the Æluroidea, and especially in the cat tribe (Felidae). In the genus Felis the dental formula is i ³/₃ c ¹/₁ pm ³/₂ m ¹/₁, total 30. The incisors are very small, so as not to interfere with the action of the large canines, the lower carnassial is reduced to simply the bilobed blade (fig. 84, IV), and the cheek teeth are greatly subordinated to the carnassial. The extinct Machaerodus has the upper canines comparable in size to those of the Walrus.

The Civets and Hyaenas have a dentition allying them closely to the cats. The hyaena-like Proteles has, however, the grinding teeth greatly reduced.

In the Cynoidea[163] the general dentition is i ³/₃ c ¹/₁ pm ⁴/₄ m ²/₃, total 42. This differs from the regular mammalian dentition only in the absence of the last upper molar. The upper carnassial tooth (fig. 84, II.) consists of a larger middle and smaller posterior lobe with hardly any trace of an anterior lobe. The lower carnassial (fig. 84, V.) is typical, consisting of a bilobed blade with inner cusp and posterior talon.

The dentition of the Cynoidea is most closely linked with that of the Arctoidea by means of fossil forms.

In the Arctoidea the dentition is not so typically carnivorous as in the Æluroidea and Cynoidea. In the bears, Ursidae, the molars have broad flat tuberculated crowns (fig. 85). The dental formula in Ursus is i ³/₃ c ¹/₁ pm ⁴/₄ m ²/₃, total 42. The upper carnassial (fig. 84, III.) differs from that of the Æluroidea and Cynoidea in having no inner lobe supported on a third root. In the large group of Mustelidae there are generally two molars in the lower and one in the upper jaw. The grinding teeth commonly have large, flattened, more or less tuberculated crowns, and the upper molar may be as large or much smaller than the carnassial.

In the Creodonta there are no specially differentiated carnassial teeth.

In the Pinnipedia the dentition differs considerably from that of the Carnivora vera. The milk dentition is always vestigial, and the teeth are frequently absorbed before birth. There are four premolars and one molar, forming an uniform series of cheek teeth, all of which except in the Walrus have compressed and pointed, never flattened, crowns. There is no special carnassial tooth, and the incisors are always fewer than ³/₃. In Otaria the dentition is

i ³/₂ c ¹/₁ pm ⁴/₄ m 1 or ²/₁, total 34 or 36.

In the Walrus the upper canines form immense tusks. The other teeth are all small and one-rooted, and the molars have flat crowns. In the true seals the dentition is strikingly piscivorous, the cheek teeth often having accessory cusps (fig. 86).

The Insectivora are diphyodont and heterodont, having well-developed rooted teeth. The canines are usually weak, the incisors pointed, and those of the two jaws often meet like a pair of forceps. The crowns of the molars are characteristically studded with short cusps. Some genera, such as Gymnura and the mole, Talpa, have the regular mammalian dentition. In the hedgehog, Erinaceus, the dentition is

i ³/₂ c ¹/₁ pm ³/₂ m ³/₃, total 36.

In the genus Sorex (Shrews) the teeth differ in the following two marked respects from those of most other Monodelphia, (1) they are monophyodont, (2) the lower incisors sometimes become fused to the jaws. Most Insectivora have square molar teeth, but in Potamogale, Chrysochloris, Solenodon and the Centetidae the molar teeth are triangular in section. Four molars occur in Centetes.

In the aberrant genus Galeopithecus the dentition is i ²/₃ c ¹/₁ pm ²/₂ m ³/₃, total 34. The upper incisors are placed at some distance from the anterior end of the jaw, and the outer upper incisors and canines of both jaws have two roots,—a very unusual character. The lower incisors are deeply grooved or pectinated in the same way as are the lower incisors of Procavia. The upper incisors and canines of both jaws bear many cusps, and are very similar in appearance to the cheek teeth of some Seals.

The dentition of the Chiroptera is diphyodont and heterodont, and the dental formula never exceeds

i ²/₃ c ¹/₁ pm ³/₃ m ³/₃, total 38.

The milk teeth are very slender and have sharp recurved cusps; they are quite unlike the permanent teeth. The permanent teeth are of two types. In the Insectivorous forms the molar teeth are cusped, and resemble those of Insectivora. In the blood-sucking Vampire bat Desmodus, the teeth are peculiarly modified; the canines and the single pair of upper incisors are much enlarged and exceedingly sharp, while all the other teeth are much reduced in size.

In the Frugivorous bats the molar teeth have nearly always smooth crowns. The dental formula in the chief genus Pteropus is i ²/₂ c ¹/₁ pm ³/₃ m ²/₃, total 34.

The Primates have a diphyodont and heterodont dentition, generally of an omnivorous type, with cheek teeth adapted for grinding. The incisors are generally ²/₂, and the molars, except in the Hapalidae, are ³/₃. In the Lemurs the upper canines are large, and the lower incisors slender and directed almost horizontally forwards. The Aye Aye, Chiromys, has the following singular dentition: i ¹/₁ c ⁰/₀ pm ¹/₀ m ³/₃, total 18. The incisors much resemble those of rodents having persistent pulps, and enamel only on the anterior face.

In Man and in the Anthropoid and Old World Apes the dental formula is always i ²/₂ c ¹/₁ pm ²/₂ m ³/₃, total 32.

In the Cebidae there is an extra premolar in each jaw bringing the number up to 36. In the Hapalidae, as in the Cebidae, there is a third premolar, but the molars are reduced to ²/₂. Man is the only Primate that has the teeth arranged in a continuous series. In all the others there is a gap or diastema of larger or smaller size between the incisors and canines. In all except man also the canines are enlarged, especially in the males.

The Exoskeletal structures of mammals may be summarised in the following table:

I. Epidermal exoskeletal structures.

II. Dermal exoskeletal structures.

ENDOSKELETON.

Vertebral Column.

Cervical Vertebrae.

The cervical vertebrae of all mammals have certain characters in common. However long the neck may be, the number of cervical vertebrae, with very few exceptions, is seven. Movable ribs are generally absent, and if present are small and do not reach the sternum. The transverse processes are generally wide but not long, and are perforated near the base by the vertebrarterial canals, through which the vertebral arteries pass; they generally bear downwardly-directed inferior lamellae which are sometimes as in the seventh human cervical seen to ossify from centres distinct from those forming the rest of the transverse process, and are really of the nature of ribs. The atlas and axis always differ much from the other vertebrae.

We may pass now to the special characters of the cervical vertebrae in the different groups. In Monotremes and Marsupials the number of cervical vertebrae is always seven. With the exception of the atlas of Echidna the cervical vertebrae of Monotremes are without zygapophyses. In Monotremes the transverse processes ossify from centres distinct from that forming the body, and remain suturally connected with the rest of the vertebra until the adult condition is reached. The method of the ossification of the atlas in Marsupials varies considerably, thus in some forms such as the Wombats (Phascolomys) there is an unossified gap in the middle of the inferior arch of the atlas, which may remain permanently open; in Thylacinus this gap is filled up by a distinct heart-shaped piece of bone, while in Didelphys and Perameles the atlas is ossified below in the same way as in other mammals. In Notoryctes the second to sixth cervical vertebrae are ankylosed together.

The cervical vertebrae of the Edentata have some remarkable peculiarities. In the three-fingered Sloth, Bradypus, there are nine cervical vertebrae, all except the last of which have their transverse processes perforated by the vertebrarterial canals. In a two-fingered sloth, Choloepus hoffmanni, there are only six cervical vertebrae. In the Megatheriidae, Anteaters (Myrmecophagidae), Pangolins (Manidae), and Aard Varks (Orycteropodidae), the cervical vertebrae are normal, but in the Armadillos (Dasypodidae), and still more in the Glyptodonts, several of them are commonly fused together. The fusion affects not only the centra, but also the neural arches, so that the neural canals form a continuous tube.

In the Glyptodonts there is a complex joint at the base of the neck to allow the partial retraction of the head within the carapace. This arrangement recalls that in Tortoises.

As a rule the Sirenia possess seven short cervical vertebrae, not fused together and not presenting any marked peculiarities. In Manatus however there are only six cervical vertebrae and they are very variable.

In the Cetacea there are invariably seven cervical vertebrae, but they are always very short and are frequently even before birth fused together by their centra into one continuous mass (see fig. 67). Sometimes the last one or two are free. In the Rorquals (Balaenoptera) however, the cervical vertebrae are quite separate and distinct (fig. 87), and in the fluviatile Odontoceti, Platanista, Inia, and Pontoporia, and also in Beluga and Monodon, though very short they are free. In Physeter the first vertebra is free while the others are fused. An odontoid process is not commonly present even in Cetaceans with free cervical vertebrae, but a very short one occurs in the Rorquals. The cervical vertebrae of Rorquals give off on each side two transverse processes (fig. 87, 3 and 4) which enclose between them a wide space. These processes are not completely ossified till the animal is adult.

In all Ungulata the number of cervical vertebrae is seven. Among the Artiodactyla two forms of the odontoid process of the axis occurs; in the Suina and Tragulina it is conical, in the Ruminantia and Tylopoda it is spout-like (fig. 88, 4). The atlas in the Suina and to a less extent in the Ruminantia has long flattened transverse processes, and the remaining cervical vertebrae are opisthocoelous. Those of the Giraffe and Llama (fig. 103) are noticeable for their great length. In the Tylopoda the posterior half of the vertebrarterial canal is confluent with the neural canal.

The Perissodactyla have remarkably opisthocoelous cervical vertebrae. Those of Macrauchenia have the posterior half of the vertebrarterial canal confluent with the neural canal as in Tylopoda. In the Proboscidea they are short flattened discs slightly opisthocoelous; the axis and seventh vertebra and to a less extent the sixth have high neural spines.

In the Rodentia the atlas generally has broad wing-like transverse processes, and the axis a large and long neural spine, while the odontoid process is much developed. In the Jerboas (Dipus) all the cervical vertebrae except the atlas are fused together, a condition recalling that in armadillos.

In the Carnivora the wings of the atlas are well developed (fig. 69, A, 1), and it is deeply cupped for articulation with the condyles of the skull. The axis has a long odontoid process and a high compressed neural spine (fig. 69, B, 4). The third to sixth cervical vertebrae have large transverse processes with prominent perforated inferior lamellae, whose ventral margins in the third and fourth vertebrae diverge as they pass backwards, while in the fifth they are parallel and in the sixth convergent. The transverse processes of the seventh vertebra have no inferior lamellae and are not perforated. Metapophyses are often developed.

In the Insectivora the cervical vertebrae vary considerably. The neural spines except in the case of the axis are generally very small and in the Shrews and Moles the neural arches are exceedingly slender.

In the Chiroptera all the cervical vertebrae are broad and short with slender neural arches.

Primates. In Man the cervical vertebrae have short blunt transverse processes and small often bifid neural spines. The neural and vertebrarterial canals are large. The atlas forms a ring surrounding a large cavity, and has a very slender inferior arch and small transverse processes. Traces of a pro-atlas have been described in Macacus and Cynocephalus. The axis has a prominent spine and odontoid process and short transverse processes. In most Primates the cervical vertebrae are very similar to those of man, but the inferior lamellae of the transverse processes are better developed. In the Anthropoid Apes the neural spines are as a rule much elongated.

Thoraco-lumbar, or Trunk vertebrae.

In the Monotremata there are nineteen thoraco-lumbar vertebrae, sixteen (Echidna) or seventeen (Ornithorhynchus) of which bear ribs. The transverse processes are very short and do not articulate with the ribs, which are united to the centra only.

In the Marsupialia there are always nineteen thoraco-lumbar vertebrae, thirteen of which generally bear ribs. The lumbar vertebrae frequently have large metapophyses and anapophyses, these being specially well seen in the Kangaroos and Koala (Phascolarctus).

The Edentata are very variable as regards their trunk vertebrae. The two genera of Sloths differ much as regards the number, for while Bradypus has only nineteen, fifteen or sixteen of which bear ribs, Choloepus has twenty-seven, twenty-four of which are thoracic, and bear ribs. In Bradypus a small outgrowth from the transverse process articulates with the neural arch of the succeeding vertebra. In both genera the neural spines are all directed backwards.

In the Megatheriidae as in the sloths the neural spines are all directed backwards, and in the lumbar region additional articulating surfaces occur, better developed than are those in Bradypus.

In the ant-eaters (Myrmecophagidae) there are seventeen or eighteen thoraco-lumbar vertebrae, all of which except two or three bear ribs. The posterior thoracic and anterior lumbar vertebrae articulate in a very complex fashion, second, third, and fourth pairs of zygapophyses being progressively developed in addition to the ordinary ones, as the vertebrae are followed back.

In the Armadillos the lumbar vertebrae have long metapophyses which project upwards and forwards and help to support the carapace. In Glyptodon almost all the thoraco-lumbar vertebrae are completely ankylosed together.

In the Manidae there are no additional zygapophyses but the normal ones of the lumbar and posterior thoracic regions are very much developed, the postzygapophyses being semi-cylindrical and fitting into the deep prezygapophyses of the succeeding vertebra.

In the Sirenia the number of lumbar vertebrae is very small; in the dugong there are nineteen thoracic and four lumbar, and in the manatee seventeen thoracic and two lumbar.

In the Cetacea the number of thoracic vertebrae varies from nine in HyperoÖdon to fifteen or sixteen in Balaenoptera, and the number of lumbar vertebrae from three in Inia to twenty-four or more in Delphinus. The lumbar vertebrae are often very loosely articulated together and the zygapophyses sometimes as in the Dolphins are placed high up on the neural spines. The centra are large, short in the anterior region but becoming longer behind. The epiphyses are prominent, and so are the neural spines and to a less extent the metapophyses. The transverse processes are well developed, anteriorly they arise high up on the neural arch, but when the vertebral column is followed back they come gradually to be placed lower down, till in the lumbar region they project from the middle of the centra. This can be well traced in the Porpoise (Phocaena). In the Physeteridae the transverse processes of the anterior thoracic vertebrae are similar to those of most Cetacea, but when followed back, instead of shifting their position on the vertebrae, they gradually disappear, and other processes gradually arise from the point where the capitulum of the rib articulates.

Ungulata. In the Ungulata vera the thoraco-lumbar vertebrae are slightly opisthocoelous. The anterior thoracic vertebrae commonly have exceedingly high backwardly-projecting neural spines (fig. 89, 1); but those of the lumbar and posterior thoracic vertebrae often point somewhat forwards so that the spines all converge somewhat to a point called the centre of motion (cp. fig. 101). In the Artiodactyla there are always nineteen thoraco-lumbar vertebrae, and in the Perissodactyla twenty-three.

Procavia sometimes has thirty thoraco-lumbar vertebrae, a greater number than occurs in any other terrestrial mammal; twenty-two of these are thoracic and eight lumbar. In Phenacodus the convergence of the neural spines to a centre of motion is well seen.

In the Proboscidea there are twenty-three thoraco-lumbar vertebrae, of which nineteen or twenty bear ribs.

In the Rodentia there are generally nineteen thoraco-lumbar vertebrae but occasionally the number rises as high as twenty-five. In the Hares (Leporidae) the number is nineteen, twelve or thirteen of which are thoracic. The anterior thoracic vertebrae have short centra and high backwardly-directed neural spines, the lumbar vertebrae have large forwardly- and downwardly-directed transverse processes with expanded ends. Metapophyses, anapophyses and hypapophyses are all present. In the Agouti (Dasyprocta) the convergence of the neural spines to a centre of motion is very strongly marked.

In the Carnivora the trunk vertebrae are nearly always twenty or twenty-one in number; in the genera Felis and Canis thirteen of these are thoracic and seven lumbar. The anterior thoracic vertebrae have long backwardly-projecting neural spines, while the posterior thoracic and lumbar vertebrae have shorter and thicker neural spines which project slightly forwards. In the Pinnipedia there is no change in the direction of the neural spines, and anapophyses are but little developed.

In the Insectivora the number of trunk vertebrae varies much from nineteen—thirteen thoracic and six lumbar—in Tupaia, to twenty-four—nineteen thoracic and five lumbar—in Centetes. The development of the various processes varies in accordance with the habits of the animals, being great in the active forms, slight in the slowly moving or burrowing forms. In Talpa and Galeopithecus the intervertebral discs of the thoraco-lumbar region instead of being cartilaginous have ossified forming inter centra, a condition met with in very few mammals.

In the Chiroptera there are seventeen or eighteen thoraco-lumbar vertebrae, eleven to fourteen of which may bear ribs. The development of processes is slight.

Among Primates the number of trunk vertebrae is generally nineteen, of which twelve to fourteen bear ribs; in man and the Gorilla and Chimpanzee the number is, however, seventeen, and in the Orang (Simia) sixteen. In some of the Lemuroidea there are as many as twenty-three or twenty-four. In most cases the neural spines converge more or less to a centre of motion, and this is especially marked in some of the Lemurs; it does not occur in man and the anthropoid apes.

Sacral and caudal vertebrae.

At the posterior end of the trunk in all mammals a certain number of vertebrae are found fused together forming the sacrum. But of these only two or three answer to the definition of true sacral vertebrae in being united to the ilia by small ribs. The others which belong to the caudal series may be called pseudosacral vertebrae. In different individuals of the same species it sometimes happens that different vertebrae are attached to the pelvis and form the sacrum. Sometimes even different vertebrae are attached to the pelvis at successive periods in the life history of the individual. This is owing to a shifting of the pelvis and has been especially well seen in man. In young human embryos the pelvis is at a certain stage attached to vertebra 30, but as development goes on it becomes progressively attached to the twenty-ninth, twenty-eighth, twenty-seventh, twenty-sixth and twenty-fifth vertebrae. As the attachment to these anterior vertebrae is gained, the attachment to the posterior ones becomes lost, so that in the adult the pelvis is generally attached to vertebrae 25 and 26. But there are no absolutely pre-determined sacral vertebrae, as sometimes the pelvis does not reach vertebra 25, remaining attached to vertebrae 26 and 27; sometimes it becomes attached even to vertebra 24. This shifting of the pelvis is seen in Choloepus in a more marked degree even than in man.

Of the Monotremata, Ornithorhynchus has two sacral vertebrae ankylosed together, while Echidna has three or four[164].

In Marsupialia as a rule only one vertebra is directly united to the ilia, but one or two more are commonly fused to the first. In the Wombats there may be as many as four or five vertebrae fused together in the sacral region. In Notoryctes there is extensive fusion in the sacral region, six vertebrae, owing mainly to the great development of their metapophyses, being united with one another, and with the ilia, and the greater part of the ischia.

In most Edentata there is an extensive fusion of vertebrae in the sacral region. This is especially marked in the Armadillos and Megatheriidae, and to a less extent in the Sloths and Aard Varks.

In the Sirenia the vestigial pelvis is attached by ligament to the transverse processes of a single vertebra, which hence may be regarded as sacral.

In Cetacea there is no sacrum, the vestigial pelvis not being connected with the vertebral column.

In most Ungulata the sacrum consists of one large vertebra united to the ilia, and having a varying number of smaller vertebrae fused with it behind.

The same arrangement obtains in most Rodentia, but in the Beavers (Castoridae) all the fused vertebrae are of much the same size, the posterior ones having long transverse processes which nearly meet the ilia.

In Carnivora there may be two sacral vertebrae as in the Hyaena, three as in the Dog, four or five as in Bears and Seals.

In Insectivora from three to five are united, while in many Chiroptera all the sacral and caudal vertebrae have coalesced. Among Primates, in Man and Anthropoid Apes there are usually five fused vertebrae forming the sacrum, but of these only two or three are connected to the ilia by ribs. In most of the other Anthropoidea there are two or three fused vertebrae, and in the Lemuroidea two to five.

Free Caudal Vertebrae. The free caudal vertebrae vary greatly in number and character. When the tail is well developed, the anterior vertebrae are comparatively short and broad, with well-developed neural arches and zygapophyses; but as the tail is followed back, the centra gradually lengthen and become cylindrical, and at the same time the neural arches and all the processes gradually become reduced and disappear, so that the last few vertebrae consist of simple rod-like centra. Chevron bones are frequently well-developed.

Of the Monotremes Echidna has twelve caudal vertebrae, two of which bear irregular chevron bones. In Ornithorhynchus there are twenty or twenty-one caudal vertebrae with well-developed hypapophyses, but no chevron bones.

In Marsupials there is great diversity as regards the tail. In the Wombat and Koala the tail is small and without chevron bones. In most other Marsupials it is very long, having sometimes as many as thirty-five vertebrae in the prehensile-tailed opossums. In the Kangaroos the tail is very large and stout. Chevron bones are almost always present, and in Notoryctes are large and expanded.

Most Edentates have large tails with well-developed chevron bones. The length of the tail varies greatly from the rudimentary condition in Sloths to that in the Pangolins, one of which has forty-six to forty-nine caudal vertebrae—the largest number in any known mammal. Chevron bones are much developed, sometimes they are Y-shaped, sometimes as in Priodon, they have strong diverging processes. The caudal vertebrae of Glyptodonts, though enclosed in a continuous bony sheath, have not become ankylosed together.

The Sirenia have numerous caudal vertebrae with wide transverse processes. In the Cetacea also the tail is much developed, and the anterior vertebrae have large chevron bones and prominent straight transverse processes; the posterior caudal vertebrae, which in life are enclosed in the horizontally expanded tail fin, are without transverse processes.

In Ungulata the tail is simple, formed of short cylindrical vertebrae, which in living forms are never provided with chevron bones. The number of caudal vertebrae varies from four, sometimes met with in Procavia, to thirty-one in the Elephant. The tail is exceedingly long in Anoplotherium and in Phenacodus, in which there are thirty caudal vertebrae.

In Rodentia the tail is variable. In the Hares, Guinea pig (Cavia) and Capybara it is very small, in Pedetes and the Beaver it is very long and has well-developed chevron bones.

Most of the Carnivora except the Bears and Seals have very long tails, the greatest number of vertebrae, thirty-six, being met with in Paradoxurus. Bears have only eight to ten caudal vertebrae. Chevron bones are not often much developed.

In Insectivora the tail is very variable as regards length, the number of vertebrae varying from eight in Centetes to forty-three in Microgale.

In Chiroptera the tail is sometimes quite rudimentary, and as in Pteropus, composed of a few coalesced vertebrae, sometimes it is formed of a large number of slender vertebrae.

In Primates also the tail is very variable. In Man all the four caudal vertebrae are rudimentary and are fused together, forming the coccyx. In the Anthropoid apes, too, there are only four or five caudal vertebrae. In many monkeys of both the eastern and western hemispheres the tail is very long, having thirty-three vertebrae in Ateles, in which genus it is also prehensile. Chevron bones are present in all Primates with well-developed tails. In the Lemuroidea the number of caudal vertebrae varies from seven to twenty-nine.