[1] The skin consists of an outer layer of epiblastic origin, the epidermis, and an inner layer of mesoblastic origin, the dermis. The epidermis is divided into two principal layers, an outer one, the horny layer or stratum corneum, and an inner one, the stratum Malpighii. The innermost part of the stratum corneum is distinguished as the stratum lucidum, and the outermost part of the stratum Malpighii as the stratum granulosum.

[2] The enamel of the pharyngeal teeth of some Teleosteans is hypoblastic in origin.

[3] See also p. 71.

[4] It is usual to regard the clavicle as a membrane bone, but KÖlliker has shown that in rabbit embryos of about the 17th day it is cartilaginous.

[5] In compiling these paragraphs on Histology, free use has been made of Klein and Noble Smith's Atlas of Histology, the small Histologies of Klein and SchÄfer, Huxley's Elementary Physiology, and Lloyd Morgan's Animal Biology.

[6] See Huxley's Elementary Physiology, Revised edition, London, 1886, p. 180.

[7] Strictly speaking the jaws, visceral skeleton, ribs and sternum do not form part of the axis, but it is convenient to group them as parts of the axial skeleton.

[8] F.M. Balfour, Comparative Embryology, vol. II., London, 1881, p. 465. W.K. Parker and G.T. Bettany, The Morphology of the Skull, London, 1877.

[9] Sometimes also called ectethmoids or parethmoids.

[10] The proximal end of anything is the one nearest the point of origin or attachment, the distal end is the one furthest from the point of origin or attachment.

[11] W.K. Parker, A Monograph of the Shoulder Girdle and Sternum, Ray Soc. London, 1868.

[12] See R. Wiedersheim, Zeitschr. wiss. Zool. vol. LIII. suppl. p. 43, 1892.

[13] G. Baur, BeitrÄge zur Morphogenie des Carpus und Tarsus der Vertebraten, Theil 1. Batrachia. Jena, 1888, and Amer. Natural., vol. XIX. 1885 (several papers).

[14] This account is based on Chapter XX. of Flower's Osteology of the Mammalia. London 1876.

[15] A. Smith Woodward, Catalogue of Fossil Fishes in the British Museum, Part II., Introduction, p. xii.

[16] This classification of reptiles is mainly based on that of Lydekker (Catalogue of Fossil Reptiles in the British Museum) but in some respects that of von Zittel has been followed.

[17] This classification of birds is essentially that of Gadow and Selenka in Bronn's Classen und Ordnungen des Thierreichs, Band vi., Abth. iv., VÖgel. Leipzig, 1891.

[18] The classification adopted is almost entirely that given in Flower and Lydekker's Mammals Living and Extinct. London, 1891.

[19] The name Balanoglossus is used here in its widest sense to include all the Enteropneusta.

[20] See W. Bateson, Quart. J. Micr. Sci. n. s. vol. XXIV. 1884, p. 208 and later; also E.W. Macbride, Ibid. vol. XXXVI. 1894, p. 385.

[21] See p. 52.

[22] A.T. Masterman, P.R. Soc. Edinb. 1895-96, p. 59; and Anat. Anz. 1896, p. 266.

[23] See E. Ray Lankester, Quart. J. Micr. Sci. vol. XXIX. n. s. 1889, p. 365. W.B. Benham, Ibid. vol. XXXV. n. s. 1893, p. 97. J.W. Kirkaldy, Ibid. vol. XXXVII. n. s. 1895, p. 303. The last-named writer divides the genus into three subgenera.

[24] See W.K. Parker On the skeleton of the Marsipobranch fishes, Phil. Trans. 1883, London.

[25] See A. Smith Woodward, Catalogue of Fossil Fish in the British Museum, Part II., 1891. A. Smith Woodward, Nat. Sci. vol. I. 1892, p. 596.

[26] See R.H. Traquair, Ann. Nat. Hist., ser. 6, vol. II. 1888, p. 485.

[27] See p. 17.

[28] R.H. Traquair, Ann. Nat. Hist. vol. VI. 1890, p. 485; P. Phys. Soc. Edinb. vol. XII. 1892-93, pp. 87-94, and 312-320. A. Smith Woodward, Nat. Sci. vol. III. p. 128, 1893.

[29] See p. 79.

[30] For this and other groups of extinct fish see A. Smith Woodward, Catalogue of Fossil Fish in the British Museum, Parts I.-III. London, 1889-95.

[31] See p. 127.

[32] See B. Dean, J. Morphol. vol. IX. pp. 87-114, 1894, and Nat. Sci. vol. VIII. p. 245, 1896.

[33] A. GÜnther, Phil. Trans. vol. 161, Part II. 1871, p. 511. T.H. Huxley, "On Ceratodus and the classification of fishes," P.Z.S. 1876, p. 24.

[34] See Marshall and Hurst's Practical Zoology, 4th ed. London, 1895, p. 214.

[35] See T.J. Parker's Zootomy, London, 1884, p. 86.

[36] See W.K. Parker and G.T. Bettany, The Morphology of the Skull, London, 1877, chap. 3.

[37] T.J. Parker, Zootomy, London, 1884, p. 91.

[38] According to G. Swirski, Schultergurtel des Hechtes, Dorpat, 1880, the true coracoid is aborted, and the so-called coracoid of Teleosteans is really the precoracoid.

[39] The following general works on fishes may be referred to: Bashford Dean, Fishes, Living and Fossil, New York, 1895. A. GÜnther, An Introduction to the Study of Fishes, Edinburgh, 1880. A.A.W. Hubrecht and M. Sagemehl, Fische in Bronn's Classen und Ordnungen des Thierreichs, Band VI. Leipzig, 1876.

[40] See W.G. Ridewood, Nat. Sci. vol. VIII. 1896, p. 380. Full references are there given to the literature of the subject.

[41] See H. Gadow and E.C. Abbott, Phil. Trans. vol. 186 (1895) B. pp. 163-221.

[42] C. Hasse, Zeitschr. wiss. Zool. LVII. 1893, p. 76.

[43] C. Hasse, Das natÜrliche System der Elasmobranchier auf Grundlage des Baues und der Entwickelung ihrer WirbelsÄule, Jena, 1879 and 1885, and "Die fossilen Wirbel, Morph. Studien I.-IV.," Morphol. Jahrb. Bd. II., III. and IV. 1876-78.

[44] See H.B. Pollard, Anat. Anz. X. 1894.

[45] T.W. Bridge, "The Cranial Osteology of Amia calva," J. Anat. Physiol. norm. path. 1876, vol. XI. p. 605. R. Shufeldt, "The Osteology of Amia calva," Ann. Rep. of the Commissioner for Fish and Fisheries, Washington, 1885.

[46] A. Smith Woodward, Nat. Sci. vol. I. 1892, p. 28. Further references are here given on the literature of the subject.

[47] C. Gegenbaur, Ueber das Archipterygium, Jena Zeitschr. der Wirbelthiere, 2^e Heft, 1873, vol. 7, and Morphol. Jahrb. XXII. 1894, p. 119.

[48] The fins of Ceratodus are very variable, no two being exactly alike. Sometimes even the main axis bifurcates. See W.A. Haswell, Linn. Soc. N. S. Wales, vol. VII. 1882.

[49] Some of these views with regard to the homologies of the parts of the fins are not accepted by all anatomists.

[50] T.H. Huxley, Amphibia (Encyclopaedia Britannica).

[51] See R. Wiedersheim, Morphol. Jahrb. Bd. III. 1877, p. 459.

[52] See A. Fritsch, Fauna der Gaskohle, Prague, 1883-85-86, also writings of Cope, Credner, Huxley, H. v. Meyer, Miall.

[53] See R. Wiedersheim, Anatomie der Gymnophionen, Jena, 1879.

[54] i.e. between one vertebra and the next.

[55] See A. Ecker, Die anatomie des Frosches, Braunschweig 1864, translated by G. Haslam, Oxford, 1889, also A.M. Marshall, The Frog, 5th edition, Manchester and London, 1894.

[56] W.K. Parker, Phil. Trans. 161, 1871, p. 137, and W.K. Parker and G.T. Bettany, The Morphology of the Skull, London, 1877, p. 136.

[57] See G.B. Howes and W. Ridewood, P.Z.S., 1888, p. 141.

[58] See bottom of p. 187.

[59] O. Hertwig. Ueber das Zahnsystem der Amphibien. Arch. mikr. Anat. supplem. Bd. XI. 1875.

[60] G.A. Boulenger, P.Z.S. 1890, p. 664.

[61] See p. 15.

[62] See many papers by W.K. Parker published in the Phil. Trans. of the Royal Soc.

[63] Perhaps this bone includes supra-orbital and postorbital elements.

[64] The first digit present is sometimes regarded as the pollex, but from analogy with Anura it is probable that the pollex is the missing digit.

[65] According to Baur a distinct epi-otic is not recognisable in the reptilian skull.

[66] H. Gadow, Phil. Trans., vol. 179, 1888.

[67] See G. Baur, J. Morph., vol. I., 1887. R. Lydekker, Catalogue of the Fossil Reptilia and Amphibia in the British Museum, Parts I. & II. C.K. Hoffmann, Reptilien, in Bronn's Classen und Ordnungen des Thierreichs, Bd. VI., 3 abth. 1879-90.

[68] T.H. Huxley, Quart. J. Geol. Soc., vol. xv. p. 649, 1859. R. Owen, Catalogue of Fossil Reptiles of S. Africa in the British Museum, London, 1876. H.G. Seeley, various papers published in the P.R. Soc. London, and Phil. Trans.

[69] See pp. 281-283.

[70] An ent-epicondylar foramen is one piercing the humerus on its inner side just above the condyle.

[71] According to Hulke they should be regarded as the omosternum,—the clavicles and interclavicle being wanting.

[72] See p. 272.

[73] R. Lydekker, Nat. Sci. vol. I. p. 514, 1892. Further references are there given.

[74] The exact position of the suture between the prefrontal and postfrontal is not known.

[75] A. GÜnther, On the Anatomy of Hatteria, Phil. Trans, vol. 157, 1867, p. 595.

[76] Zygosphenes are extra articulating surfaces borne upon the anterior face of the neural arch; they fit into corresponding structures, the zygantra, which are borne on the posterior surface of the neural arch of the preceding vertebra. Ordinary zygapophyses always accompany them.

[77] See E.D. Cope, P. Amer. Phil. Soc. vol. xxx. p. 185.

[78] See W.K. Parker, Phil. Trans. vol. 170, 1879, p. 595.

[79] See p. 281.

[80] Often called the columella cranii.

[81] See C.K. Hoffmann, in Bronn's Klassen und Ordnungen des Thierreichs, Bd. VI., 3 abth. 1885-90.

[82] Some anatomists consider that the closing in of the brain case in front is entirely due to the frontals and parietals.

[83] E.D. Cope, Rep. U.S. Geol. Surv., 1875, vol. II., The vertebrata of the Cretaceous formations of the west. E.D. Cope, P. Boston Soc. 1862, XII. p. 250. O.C. Marsh, Amer. J. Sci., 1872, vol. 3. R. Owen, Quart. J. Geol. Soc., 1877, and 1878.

[84] J.W. Hulke, Presidential address to the Geol. Soc. of London, 1883 and 1884. O.C. Marsh, many papers in the Amer. J. Sci. from 1878 onwards, also in the Geol. Mag. R. Owen, History of British fossil reptiles: Dinosauria (Palaeont. Soc.).

[85] The diagnostic characters of the different groups of Dinosaurs are in the main those given by von Zittel.

[86] See O.C. Marsh, Amer. J. Sci. (3), vol. 48, 1894, p. 85.

[87] See C.B. BrÜhl, Das Skelet der Krokodiliden, Wien, 1862. C.K. Hoffmann in Bronn's Klassen und Ordnungen des Thierreichs, Bd. VI. Abth. III. 1881-85. T.H. Huxley, Proc. Linn. Soc. (Zoology) 1860 vol. IV. p. 1. R. Owen, History of British fossil Reptiles. Crocodilia (Palaeont. Soc.). A. Smith Woodward, Geol. Mag. 1885, 3rd dec. II. p. 496. A. Smith Woodward, Proc. of Geologists' Assoc. vol. IX. p. 288, 1886.

[88] See H.G. Seeley, On the Organisation of the Ornithosauria, Journ. Linn. Soc. (Zoology) vol. XIII. p. 84. K.A. Zittel, Ueber Flugsaurier aus dem lithographischen schiefer, Palaeontograph. XXIX. p. 49.

[89] Another view commonly held is that the neural and costal plates are respectively formed by the expanded neural arches and ribs.

[90] Free use has been made of L.C. Miall's Studies in Comparative Anatomy, I., The Skull in Crocodilia, London, 1878. See also W.K. Parker, Tr. Z.S., vol. XI. 1885, p. 263.

[91] See pp. 214 and 215.

[92] These terms are defined on p. 199.

[93] E.T. Newton, Phil. Trans. vol. CLXXXIV, B, p. 431 (1893).

[94] M. FÜrbringer, Untersuchungen zur Morphologie und Systematik der VÖgel, I. and II. Amsterdam, 1888. Cf. H. Gadow, Nature, XXXIX. 1888, pp. 150 and 177.

T.H. Huxley, "On the classification of birds." P.Z.S., London, 1867.

E. Selenka and H. Gadow, VÖgel in Bronn's Classen und Ordnungen des Thierreichs 1869-1890.

[95] See p. 283.

[96] R. Owen, Phil. Trans., vol. CLIII., p. 33; 1863. T.H. Huxley, P. R.S., vol. XVI., p. 243; 1868. C. Vogt, Rev. Scient., ser. 2, tom. 9, p. 241; 1879. C.H. Hurst, Nat. Sci., vol. III., p. 275; 1893; vol. VI., pp. 112, 180, 244; 1895. W.P. Pycraft, Nat. Sci., vol. V., pp. 350 and 437; 1894; and vol. VIII., p. 261; 1896.

[97] According to Hurst the fourth and fifth digits are also present.

[98] See C.W. Andrews, P.Z.S., 1894, p. 108.

[99] See T.J. Parker, Tr. Zool. Soc. London, vol. XIII., pt. 2, 1895, and F.W. Hutton, several papers in Tr. N. Zealand Inst., 1893 and 1895.

[100] See O.C. Marsh. Odontornithes. A monograph of the extinct toothed birds of N. America. New Haven, 1880.

[101] See R.S. Wray, P.Z.S., 1887, p. 343.

[102] Often called the manubrium, but not homologous with the manubrium of the mammalian sternum.

[103] Cp. fig. 63.

[104] See p. 28.

[105] W.K. Parker, Phil. Trans. vol. 179, p. 385, 1888; and Ibis, 1888, p. 124.

[106] See T.H. Huxley, "On the Classification of Birds," P.Z.S. 1867.

[107] B. Lindsay, P.Z.S. 1885, p. 684.

[108] This is Gadow's view; according to Huxley the quadrate forms the malleus; according to Baur it forms the zygomatic process of the squamosal, and according to Broom the interarticular mandibular cartilage.

[109] According to Leche, Morphol. Jahrb. XIX. p. 502, the molar teeth belong morphologically to the first series, i.e. they are milk teeth without vertical successors.

[110] The researches of Bateson, P.Z.S. 1892, p. 102, have shown that cases of individual variation in the number of teeth are common.

[111] Baur, however, suggests (Anat. Anz. vol. IV. 1889), that a tibial sesamoid found in Procavia, many rodents, edentates and Ornithorhynchus is a vestigial tibiale, and that the astragalus is the intermedium.

[112] This perforation of the acetabulum in Echidna is a secondary character occurring late in development, and consequently is not of phylogenetic importance.

[113] See R. Owen, "Monograph of the Fossil Mammalia of the Mesozoic Formation," Pal. Soc. Mon. 1871.

H.F. Osborn, "Structure and Affinities of Mesozoic Mammals," J. of Philad. Acad. 1888, vol. IX.

O.C. Marsh, "Jurassic Mammals," Amer. J. Sci. 1878 et seq.

[114] See Oldfield Thomas, Brit. Mus. Cat. of Marsupialia and Monotremata (1888).

[115] W. KÜkenthal, Anat. Anz. VI. p. 364, 1891. C. RÖse, Anat. Anz. VII. p. 639.

[116] These bones however have no connection with the marsupium, being nearly equally developed in both male and female. They are simply sesamoid bones forming ossifications in the inner tendon of the external oblique muscle, and are developed as supports for the abdominal wall. Very similar structures have been independently developed in various Amphibians, Reptiles and monodelphian Mammals. See W. Leche, Biol. FÖren. III. p. 120.

[117] See H. Gadow, P.Z.S. 1892, p. 361.

[118] See W.H. Flower, "On the Mutual Affinities of the Animals composing the order Edentata," P.Z.S. 1882, p. 358. For the fossil Edentates of N. America see E. Cope, Amer. Natural. 1889; for those of S. America see various papers by F. Ameghino, H. Burmeister and R. Owen. Also T.H. Huxley, "On the Osteology of Glyptodon," Phil. Trans. 1865.

[119] See J.F. Brandt, Symbolae Sirenologicae, St Petersburg, 1846, 1861, 1868.

[120] Epiphyses are fully developed in Halitherium, and traces occur in Manatus.

[121] See P.J. van Beneden and P. Gervais, OstÉographie des CÉtacÉs, 1869-80.

[122] H. Wincza, Morphol. Jahrb. XVI., p. 647.

[123] See M. Pavlow, "Études sur l'histoire palÉontologique des OngulÉs." Bull. Soc. Moscou, 1887—1890.

[124] In a plantigrade animal the whole of the foot is placed on the ground in walking. A digitigrade animal places only its toes on the ground. An intermediate condition is distinguished by the term subplantigrade.

[125] See p. 345.

[126] See p. 401.

[127] See E.D. Cope, "The Perissodactyla," Amer. Natural., 1887.

[128] See E.D. Cope, "The Condylarthra," Amer. Natural., 1884, and "Synopsis of the Vertebrates of the Puerco series," Tr. Amer. Phil. Soc., 1888. O.C. Marsh, "A new order of extinct Eocene Mammals (Mesodactyla)," Amer. J. Sci., 1892.

[129] See O. Thomas, "On the species of Hyracoidea," P.Z.S., 1892, p. 50.

[130] See E.D. Cope, "The Amblypoda," Amer. Natural., 1884 and 1885.

[131] See O.C. Marsh, "The Dinocerata," U.S. Geol. Survey, 1884, vol. X.

[132] See O.C. Marsh, Amer. J. Sci., 1875 and 1876.

[133] E.D. Cope, "The Creodonta," Amer. Natural., 1884. W.B. Scott, "Revision of the N. American Creodonta," P. Ac. Philad., 1892.

[134] See next paragraph.

[135] St G. Mivart, The Cat, London, 1881.

[136] St G. Mivart, P.Z.S., 1885.

[137] St G. Mivart, "On the Osteology of Insectivora," J. Anat. Physiol. norm. path., 1867 and 1868, and P.Z.S., 1871. G.E. Dobson, Monograph of the Insectivora, London, 1882—90.

[138] See G.E. Dobson, Brit. Mus. Catalogue of Chiroptera, 1878. See also other papers by the same author and by Oldfield Thomas.

[139] W. Ellenberger and H. Baum, Anatomie des Hundes, Berlin, 1891.

[140] T.H. Huxley, "Dental and cranial characters of the Canidae," P.Z.S. 1880.

[141] See p. 392.

[142] The dura mater is a membrane which lines the cranial cavity and is formed of tough connective tissue.

[143] These are not strictly homologous with the basi-hyal and cerato-hyal of the Dogfish.

[144] See note to p. 25.

[145] See W.H. Flower, "Remarks on the homologies and notation of the teeth in Mammalia," J. Anat. and Physiol. norm. path., Vol. III., p. 262; R. Owen, Odontography, London, 1840—45; C.S. Tomes, Manual of Dental Anatomy, London, 1876. See also H.F. Osborn, "Recent researches on succession of teeth in Mammals," Amer. Natural., XXVII., p. 493, and "Rise of Mammalia in N. America," Stud. Biol. Lab. Columb. Coll., Zool. I., no. 2.

[146] See E.B. Poulton, P.R.S., Feb. 1888, and Quart. J. Micr. Sci., Vol. XXIX. 1889; also Oldfield Thomas, P.R.S., XLVI. (1889).

[147] W.H. Flower, Phil. Trans., vol. 156, pp. 631—641, 1867; also Oldfield Thomas, Phil. Trans., pp. 443—462, 1887.

[148] C. RÖse, Anat. Anz. VII., p. 639.

[149] W. KÜkenthal, Anat. Anz. VI., p. 364.

[150] See p. 348.

[151] W. Leche, Morph. Jahrb. XX., pp. 113—142 (1893).

[152] E.C. Stirling, P.Z.S. 1891, p. 327.

[153] O. Thomas, P.Z.S., 1895, p. 870.

[154] F. Ameghino, Bull. Ac. Argen. XII. p. 437. According to H. Burmeister, Annal. Mus. Buenos Aires, III. 401 (1891), enamel does not occur, osteodentine having been mistaken for it.

[155] E. Ballowitz, Arch. Mikr. Anat. XL. p. 133.

[156] See Oldfield Thomas, P.R.S., vol. XLVII., p. 246 (1890).

[157] J. Taeker, "Fur Kenntniss der Odontogenese bei Ungulaten." Dorpat, 1892.

[158] See p. 345.

[159] See p. 345.

[160] According to H.F. Osborn, Amer. Natural., XXVI. p. 763, a number of not very closely allied forms have been included under Lophiodon.

[161] C. Earle, J. Ac. Philad., vol. IX., 1892, p. 267.

[162] Encyclopaedia Britannica, article Mammalia, p. 424.

[163] See T.H. Huxley, "The dental and cranial characters of the Canidae," P.Z.S., 1880, p. 238.

[164] G.B. Howes, Journ. of Anat. and Phys. XXVII., p. 544.

[165] The figure was drawn from a photograph and the size of the jaws relatively to the cranium is exaggerated.

[166] See W.K. Parker, "On the Structure and Development of the Skull in the Pig." Phil. Trans. pp. 289-336, 1874.

[167] See W.H. Flower, "On the value of the characters of the base of the cranium in the classification of the order Carnivora." P.Z.S. 1869, p.

[168] See W.K. Parker, Monograph of the shoulder-girdle and sternum of the Vertebrata, Ray Soc. 1868.

[169] See p. 405.

[170] See E. Lydekker, P.Z.S. 1895, p. 172.

[171] See H. Wincza, Morph. Jahr. XVI. p. 647, 1890.

[172] See K. Bardeleben, P.Z.S., 1889, p. 259.

[173] See E. Cope, "The origin of the foot structures of Ungulata," Journ. of Philad. Acad. 1874. H.F. Osborn, "The evolution of the Ungulate foot," T. Amer. Phil. Soc. 1889.

[174] See O.C. Marsh, various papers including "Fossil horses in America," Amer. Natural. 1874; "Polydactyl horses," Amer. J. Sci. 1879 and 1892. M. Pavlow, "Le dÉveloppement des EquidÉs," Bul. Soc. Moscou, 1887, and subsequent papers in the same. Osborn and Wortman, "On the Perissodactyls of the White River beds," Bull. Amer. Mus. Dec. 23rd, 1895.

[175] See H.F. Osborn, Chalicotherium and Macrotherium, Amer. Natural. 1889—91—92.

[176] See p. 409.

[177] See p. 412.