The form varies enormously, worm-like in some, comparatively short and heavy, elongate and more or less slender, or extremely gracile and almost filiform, in others. In this respect our common Grass-snake occupies a central position, and for this reason is termed a moderately slender form, anything above or below this standard being described as comparatively short or elongate. Our shortest and stoutest European Snakes are the Vipers, especially Vipera ursinii; our longest and slenderest, the Coluber and Zamenis, especially Zamenis dahlii. These extremes in both directions are, however, far surpassed by many exotic snakes, as we find on comparing, for instance, one of the African Puff-adders (Bitis), with certain Oxybelis and Leptognathus from Tropical America. The body may be somewhat rigid, as in some burrowing and ground snakes, not unlike in appearance to our Slow-worm and other limbless Lizards; or extremely flexible, as in many Pythons and Boas and in the Tree-snakes generally. This flexibility may be accompanied by a vertical compression of the body in relation with an arboreal existence, whilst sluggish snakes, such as most of the ViperidÆ, may be remarkable for the flattening of the body, which they may further increase when basking in the sun or in order to assume a more formidable appearance on the approach of an enemy. This power of flattening out the whole or the anterior part of the body is possessed by many snakes, poisonous as well as harmless, and reaches its highest degree in the Cobras of India and Africa, the expanded anterior part being known as the “hood,” from the Portuguese name “Cobra di capello.”

Thoroughly aquatic snakes are often short and heavy, but some of the marine forms, or Hydrophids, may be extremely slender, with the posterior part of the body compressed. In some of these Sea-snakes the gracility of the anterior part, or “neck,” as it has been called, contrasts very strikingly with the great girth of the body towards the tail, and suggests a limbless Plesiosaur.

The tail, the part of the body behind the transversely cleft vent, is most frequently about one-fourth or one-fifth of the total length; but it may be much shorter, even reduced to a mere stump, as in the Typhlops, or, at the opposite extreme, enter for one half in the length of the snake, as in the African Xenurophis. This organ may taper gradually to a fine point; or end abruptly, as if mutilated; or terminate in a horny spine, such as we see in some of the Typhlops or in the Australian Death-adder, Acanthophis, or in a series of horny segments which are vibrated like a rattle, as in the well-known Crotalus of America, to which we shall refer again at the end of this chapter. In some of the burrowing UropeltidÆ, the very short tail is obliquely truncated, with indurated shields above, and acts as a trowel. And, finally, the marine snakes of the subfamily HydrophiinÆ are distinguished by a strongly compressed, oar-shaped tail, with rounded vertical outline. In a few forms, arboreal or aquatic, the tail is more or less prehensile.

Males generally have a longer tail than females, and the genital organs, which are lodged in its base, cause a swelling of that region which contrasts with the more gradually tapering extremity of the female, thus affording a means of distinguishing the sexes externally in the majority of snakes.

The rudimentary hind limbs of Boid snakes, to be mentioned further on in the description of the skeleton, terminate in a claw-like horny spur, which appears on each side of the vent in the male, and sometimes also, though less distinctly, in the female. These spurs are probably of use in facilitating the pairing, an explanation which appears the more plausible from the fact that the snakes provided with them have the copulatory intromittent organs destitute of the erectile spines which are present in most others.

The head varies in shape as much as the body. Although never actually compressed, except in the rostral region, it may be very narrow and elongate, whilst in the opposite extreme it may be strongly depressed, and so broad behind as to be abruptly defined from the anterior part of the body, or “neck.” This feature is very marked in some of the ViperidÆ, and this has given rise to the incorrect generalization that poisonous snakes are distinguished from the harmless by a broad and flat head, notwithstanding the fact that some of the most dangerous, such as the Mambas, Cobras, and Kraits, have a comparatively narrow or small head, not or but slightly defined behind, whilst, on the other hand, the very opposite condition obtains in not a few of the harmless Colubrids.

Leaving the TyphlopidÆ and GlauconiidÆ aside for the present, snakes have a wide gape, cleft far beyond the vertical of the eyes, with, when closed, one or two notches in front for the passage of the protrusible, bifid tongue. In most snakes this chink is in the lower border of the rostral shield, capping the tip of the snout, and allows free passage to the whole tongue; in the Hydrophids, or Sea-snakes, there are two notches in the lower border of the rostral shield, through which only the bifid end of the tongue can be protruded. The eyes, varying from minute to enormous, are usually free from the surrounding shields, and may move under a transparent cap like a watch-glass, which appears to represent the lower eyelid of Lizards. The view as to this homology is derived from our knowledge of various conditions in certain series of Lizards of the families LacertidÆ and ScincidÆ, where we find a transparent disc appearing like a small window in the movable lower eyelid, gradually increasing in size so as to occupy the whole of the lower eyelid, which finally becomes fused with the rudimentary upper lid and loses its mobility. In Ilysia and in most of the UropeltidÆ, the transparent disc over the eye is confluent with a thick horny shield of which it occupies the middle.

The pupil is usually circular or vertical, rarely horizontal. In some forms it is difficult to decide whether it is round or vertically elliptic; in others, like the Boas and Vipers, for instance, it is decidedly vertical, and contracts to the same extent as a cat’s. In some Water-snakes, and in Sea-snakes generally, the round pupil may contract to a mere dot. The contraction of the pupil is independent on the two sides.

The snout, or the part of the head anterior to the eyes, may be short or long, rounded or pointed, depressed or compressed, sometimes projecting strongly beyond the mouth, turned up at the end, or terminating in one (Langaha) or two (Herpeton) long scaly dermal appendages. In some burrowing forms it is provided with a more or less trenchant horizontal or vertical edge. When the sides of the snout (loreal region) form an angle with the upper surface, the angle is termed the “canthus rostralis,” which may be intensified by the loreal region being concave.

The deep pits which are sometimes present on the lips or between the nostril and the eye (loreal pit) will be alluded to further on under Sensory Organs.

The nostrils are either lateral, or, in the aquatic forms, directed upwards, sometimes entirely on the upper surface of the snout.

Most snakes have a longitudinal groove on the chin (mental groove) to allow for the distension caused by the lateral movements of the rami of the lower jaw.

In the TyphlopidÆ, the head passes gradually into the vermiform body, and the small mouth is situated on the under surface of the projecting snout; the head so resembles the extremely short tail, and the mouth is so similar in shape and position to the vent, which is close to the posterior extremity of the snake, that such creatures are often believed by non-critical observers to have a head at each end. The eyes are very small, and covered over by the semi-transparent head-shields, or they may be completely concealed. There is no mental groove. It is much the same with the GlauconiidÆ, which have, however, a somewhat less abbreviated tail. In both, the nostrils often open on the lower side of the snout, which may be excavated so as to appear hooked in profile, or may be provided with a sharp cutting horizontal edge.

Snakes are covered with epidermal folds in the form of scales and shields, the shape and arrangement of which affords important characters for their classification. Dermal ossifications are absent.

The scales on the body are usually elliptic or lanceolate and imbricate, forming straight longitudinal and oblique transverse series, and they are replaced on the belly and under the tail by transverse shields mostly corresponding in number with the series of scales, and also with the vertebrÆ. The body of the TyphlopidÆ and GlauconiidÆ is uniformly covered with polished, closely adherent, rounded, overlapping, sub-equal scales, without even an indication of ventral shields. In some of the AcrochordinÆ, aberrant aquatic Colubrids, the scaling consists, above and beneath, of small juxtaposed, sometimes spinose granules, the skin being suggestive of the shagreen of sharks. In the marine snakes of the subfamily HydrophiinÆ, the ventral shields are often absent or merely indicated, and the scales are mostly juxtaposed or feebly imbricate, sometimes tetragonal or hexagonal, and occasionally studded with spinose tubercles. In the more typical Ophidia the imbricate scales may be long and narrow or short and broad, with every intermediate step between the two extremes; smooth or furnished with a longitudinal ridge or keel, or even several keels; nearly equal in size or with the median or outer series more or less enlarged, the longitudinal series in odd, rarely in even number; instead of running in longitudinal series parallel with the axis of the body, as is the rule, they are sometimes disposed obliquely, and among those in which we meet with this peculiarity several genera are further remarkable in having some of the oblique lateral scales furnished with a serrated keel, to which we shall again allude in the chapter on Habits, when dealing with the rustling sounds produced by certain snakes. The number of longitudinal series of scales on the body varies from 10 (Herpetodryas) to nearly 100 (Python, Boa); in the European species from 17 (Contia modesta) to 50 (Eryx jaculus). The scales are sometimes furnished near the end with one or two shallow impressions, termed “apical pits,” which afford indications for the distinction of genera and species; unless of a lighter or darker colour, as is often the case, these pits are not always easy to see, except in a strong light and with the aid of a powerful magnifying glass.

The ventral shields, also called “gastrosteges,” usually occupy the whole width of the belly; but they may be much narrower—in Eryx, for instance. They are sometimes bent at an angle on the sides, and this angle may even form a sharp keel, accompanied by a notch in the posterior border, corresponding to the keel, as in several of the more arboreal genera of Colubrids. The shields under the tail, termed subcaudals or “urosteges,” are sometimes similar to the ventrals, but more often disposed in pairs; in certain species or individuals some of the subcaudals are single, and the others paired. When the number of subcaudals is given in the descriptions, each pair is reckoned as one, and the conical or spine-like shield which caps the end of the tail is not included. These numbers afford important characters for the definition of species, and sometimes also for the distinction of sexes. The subcaudals are nearly always much fewer than the ventrals, but the difference is often not so great in the males as in the females, the tail of which is usually shorter in proportion to the body. It is noteworthy that in many species, if the number of subcaudals (C.) be added to that of the ventrals (V.), the total is nearly the same in the male as in the female, however much the respective numbers may differ when taken separately. The following figures may be given by way of example, taken from British specimens:

Although this rule is by no means universal, and does not apply at all to some species, it will be found to hold good in many cases, and is of interest in showing that the changes that have taken place in the vertebral column (the vertebrÆ corresponding in number to the shields), according to the sexes, have been by a modification of the character of the segments about the anal region, a conversion of trunk vertebrÆ into caudals, or vice versa. In dealing with certain species—of Vipers, for instance—it is important, for systematic purposes, to keep the counts of shields distinct for the two sexes.

The shield which covers the vent, the anal shield, is either single or divided into two.

Some snakes have the head covered with scales or small tubercles similar to those on the body, but in the great majority the lepidosis is in the form of large symmetrical juxtaposed shields, the shape, proportions, and number of which furnish some of the most important characters for the distinction of genera and species. These head-shields belong to two primarily different types, from each of which all further modifications may be regarded as derived by alteration in shape or by disintegration. The first type is that shown by the TyphlopidÆ and GlauconiidÆ, which is explained by the figure on the next page.

The rostral, which is usually the largest of the head-shields, extends to the upper surface of the head, of which it may occupy the greater part. In the GlauconiidÆ, the ocular usually borders the mouth.

As may be seen by a comparison of the first figure with the second, the arrangement of the head-shields is essentially different from that which prevails in the Colubrids and the majority of other snakes.

The second type is exemplified by the head of a member of the genus Zamenis.

In the descriptions, temporals 2 + 3 means two superposed temporals in the first row, three in the second. The internasals and the temporals, and the loreal and the preocular, are sometimes absent, and the prefrontal or the internasal may be single. One or two large shields are in rare cases present behind the parietals, and are called occipital.

A breaking up into smaller shields takes place in many snakes. In the Pythons, for instance, the frontal may be divided into two by a longitudinal cleft, and separated from the prefrontals by small shields. In some Vipers, such as V. berus and V. ursinii, in which the frontal and parietals, though reduced in size, usually preserve their primitive condition, the former is normally separated from the supraocular by a series of small shields, and the internasals and prefrontals are broken up; in these snakes the small shield or shields behind the rostral are termed “apical,” and those on the upper edge of the snout are termed “canthals.” The shield which, in Vipers, separates the rostral from the nasal is called “naso-rostral.” Allusion has been made above to the scaly dermal appendages which terminate the snout in certain genera. Some ViperidÆ are furnished with horn-like erect spines above the eyes or at the end of the snout, which add greatly to their sinistral appearance.

In Rattlesnakes each piece of the rattle, or “crotalon,” in which the tail terminates, represents a retained portion of the sloughed epidermis. This remarkable appendage looks like a number of horny rings, but it consists in reality of hollow, bell-like pieces, similar to the terminal one, or “button,” each with a circular constriction, in which the incurved free edge of the following piece fits, thus keeping the pieces together without impairing the mobility necessary to produce the rattling sound for which the apparatus is intended. At each exuviation one bell-shaped horny piece is added. The number of segments in the rattle is, therefore, not an index to age, as formerly believed; nor is it to the number of exuviations, for whilst segments are being added at the base of the apparatus the terminal ones break off and are lost. A Crotalus sixteen months old may have six pieces to the rattle if there have been six exuviations and no loss. No rattle appears ever to comprise more than about twenty pieces, even in old specimens. The size of the terminal button shows whether it was formed at birth or at any later period, no growth taking place in the horny tissue.

So far as trustworthy records are concerned, the largest snakes known, the Malay Python reticulatus and the South American Anaconda, Eunectes murinus, reach a length of 25 to 30 feet. Measurements of skins must be accepted with caution, as a skin may easily be stretched to once and a half its real length; in estimating the exact length from such a stretched skin, it is necessary to deduct the interstitial spaces showing between the scales, and about one-fourth of the scale to allow for the overlap. The smallest snake known is 4 inches long (Glauconia dissimilis). The largest European snake (Coluber quatuorlineatus) is reported to reach a length of 8 feet; the smallest (Typhlops vermicularis) does not exceed 14 inches.

I. Eyes minute, under the head-shields; mouth small, inferior; body vermiform, covered with uniform scales above and beneath; vent close to the end of the body, the extremely short tail ending in a small spine

II. Eyes very small, with vertical pupil; upper surface of head covered with small scales; ventral shields much narrower than the body; tail short, ending obtusely; subcaudals single, or mostly single; scales smooth or feebly keeled, in 40 to 50 rows

III. Eyes small, moderate, or large; ventral shields at least nearly as broad as the body; tail tapering to a point; subcaudals paired.

A. Pupil round; upper surface of head with nine large shields; no upper labial in contact with the parietal; anal shield usually divided.

1. Dorsal scales strongly keeled, with paired apical pits; a single anterior temporal.

a. Nostrils lateral; internasals broadly truncate in front.

Scales in 19 rows; normally 1 pre- and 3 postoculars; usually 7 upper labials, third and fourth entering the eye; ventrals 157-181; subcaudals 50-88

b. Nostrils directed upwards; internasals much narrowed in front.

Scales in 19 rows; normally 2 pre- and 3 or 4 postoculars; suboculars sometimes present; usually 8 upper labials, fourth or fourth and fifth entering the eye; ventrals 160-187; subcaudals 48-79

Scales in 21 (rarely 19 or 23) rows; normally 1 or 2 pre- and 2 postoculars; usually 7 upper labials, third and fourth entering the eye; ventrals 147-164; subcaudals 46-72

2. Dorsal scales smooth or feebly keeled; normally a single loreal.

a. Two or three superposed anterior temporals (very rarely one); nostril usually between two nasals.

a. A subocular below the preocular.

* Scales smooth, in 17 or 19 rows.

Two upper labials entering the eye; preocular not in contact with the frontal; scales with two apical pits; ventrals more or less distinctly angulate laterally, 160-230; subcaudals 87-131

Two upper labials entering the eye; preocular usually in contact with the frontal; scales with a single apical pit; ventrals very distinctly angulate laterally, 205-218; subcaudals 98-132

** Scales in 23 to 29 rows (usually 25 or 27), with two apical pits.

Upper labials usually separated from the eye by a series of suboculars; preocular in contact with the frontal; scales smooth; ventrals very distinctly angulate laterally, 222-258; subcaudals 77-107

Two upper labials entering the eye; preocular not in contact with the frontal; scales feebly but distinctly keeled; ventrals not angulate laterally, 195-234; subcaudals 56-90

Two upper labials entering the eye; preocular not in contact with the frontal; scales smooth or faintly keeled; ventrals not or but very obtusely angulate laterally, 172-214; subcaudals 50-80

. No subocular; scales smooth, or faintly keeled on the posterior part of the body.

* Ventrals more than 200; scales with two apical pits.

Snout obtuse; rostral broader than deep; scales in 21 or 23 rows; ventrals distinctly angulate laterally, 212-248; subcaudals 60-91

Snout obtuse; rostral broader than deep; scales in 25 or 27 rows; ventrals not angulate laterally, 222-260; subcaudals 68-90

Snout pointed, strongly projecting; rostral deeper than broad, wedged in between the internasals; scales in 25 to 29 rows; ventrals not angulate laterally, 201-220; subcaudals 48-68

** Ventrals not more than 200; scales mostly with a single apical pit.

Rostral at least as deep as broad, often wedged in between the internasals; usually 7 upper labials, third and fourth entering the eye; scales in 19 (rarely 21) rows; ventrals 153-199; subcaudals 41-70

Rostral broader than deep; usually 8 upper labials, fourth and fifth entering the eye; scales in 21 (rarely 19 or 23) rows; ventrals 170-200; subcaudals 49-72

b. A single anterior temporal; nostril in a single nasal; scales smooth, with single apical pits, in 17 rows; ventrals 150-191; subcaudals 53-78

3. Scales longitudinally grooved in the adult, in 17 or 19 rows; two loreals; canthus rostralis strongly marked; frontal very narrow, in contact with the preocular; ventrals 160-189; subcaudals 68-102

B. Pupil vertical or vertically subelliptic (sometimes appearing round in Macroprotodon).

1. Scales smooth, mostly with single apical pits; upper surface of head with nine large shields.

Frontal 11/2 to 2 times as long as broad; loreal separated from the eye by the preocular; one upper labial usually in contact with the parietal; scales in 19 to 23 (rarely 25) rows; ventrals 153-192; anal divided; subcaudals 40-54

Frontal 11/4 to 11/2 times as long as broad, much shorter than the parietals; loreal entering the eye; scales oblique, in 19 or 21 rows; ventrals 186-222; anal divided; subcaudals 48-73

Frontal 11/4 to 11/2 times as long as broad, nearly as long as the parietals; loreal entering the eye; scales oblique, in 19 or 21 rows; ventrals 203-235; anal entire; subcaudals 54-70

2. Scales keeled, with two apical pits; anal shield entire.

a. No pit between the nostril and the eye; upper head-shields small, if present; nasal separated from the rostral by a naso-rostral; eye separated from the upper labials by suboculars.

a. Snout not turned up at the end; supraocular usually extending posteriorly beyond the vertical of the posterior border of the eye; frontal and parietal shields usually well developed; usually a single series of scales between the eye and the upper labials.

Snout obtusely pointed, flat above, or with the canthus slightly raised; rostral usually in contact with a single apical shield, rarely with two; 6 to 9 upper labials, usually 7 or 8; scales in 19 rows, rarely 21; ventrals: ? 120-135, ? 125-142

Snout pointed, with raised canthus; rostral in contact with a single apical shield; 8 or 9 upper labials; scales in 21 rows, rarely 19; ventrals: ? 130-148, ? 130-150

Snout truncate or broadly rounded, flat above or with slightly raised canthus; rostral in contact with two apical shields, rarely with one; 8 or 9 upper labials; scales in 21 rows, rarely 19 or 23; ventrals: ? 132-150, ? 132-158

. Snout usually more or less turned up at the end or produced into a scaly dermal appendage; supraocular not extending posteriorly beyond the vertical of the posterior border of the eye; frontal and parietals often absent or very small; 2 or 3 series of scales between the eye and the upper labials; 9 to 13 upper labials; scales in 21 or 23 rows, rarely 19 or 25.

Snout simply turned up, the raised portion bearing 2 or 3 scales; rostral not more than once and a half as deep as broad; ventrals: ? 134-158, ? 141-169

Snout simply turned up or produced into a small appendage, the raised portion with 5 or 6 (rarely 3) scales; rostral 11/2 to 2 times as deep as broad; ventrals: ? 125-146, ? 135-147

Snout produced into an appendage covered with 10 to 20 scales; rostral not reaching the summit of the rostral appendage; ventrals: ? 133-161, ? 135-163

?. Snout not turned up at the end; supraocular narrow or broken up into several small shields; upper surface of head with small, usually keeled scales; two or three series of scales between the eye and the upper labials; scales in 23 to 27 rows, usually 25; ventrals: ? 151-177, ? 153-180

b. A pit between the nostril and the eye; upper surface of head with 9 large shields; nasal in contact with the rostral; third upper labial entering the eye; scales in 23 rows; ventrals 149-174; subcaudals 31-44