The eleven sub-classes of the Reptilia present so many important differences that it is not advisable to give here a further general account of their structure. The diagrammatic figures A to T on pp. [280], [281], representing various types of skulls, are intended to explain their chief modifications, all referable to Proreptilian and to certain Theromorphous conditions. One of the most important features is that the mandible, which is always composed of many pieces (cf. Fig. 142, p. [550]), is invariably carried by the quadrate bone. Diagrams of the generalised skulls of a Bird and a Mammal have been added for comparison.
Fig. 56.–Composition of vertebrae of Reptiles, illustrated by the first and second cervical vertebrae. (1) Atlas (first cervical) and axis (second) vertebra of Crocodilus. (2) Atlas and axis of Metriorhynchus, a Jurassic Crocodile. (3) Analysis of the first two cervical vertebrae of a Crocodile; 2, second basiventral complex or "intercentrum" continued upwards into the meniscus or intervertebral pad. (4) Diagram of the fundamental composition of a Reptilian vertebra; compare this and (6) with Fig. 1 (8 and 9) on p. [13]. (5) The first three cervical vertebrae of Sphenodon. (6) Trunk-vertebrae of Eryops, a Permian Proreptile; typically temnospondylous; cp, articular facet of the capitulum of a rib. (7) The complete atlas of an adult Trionyx hurum; the second basiventral (intercentrum) is attached to the posterior end of the first centrum, which, not being fused with the second centrum, is not yet an odontoid process. (8) The complete atlas of an adult Trionyx gangeticus; still typically temnospondylous. (9) The first and second cervical vertebrae of an adult Platemys. (10) The complete atlas of a Chelys fimbriata. Az, Anterior zygapophysis; B.D, basidorsal; B.V, basiventral; C1, C2, C3, first, second, and third centra, formed by the interventralia; Cp1, Cp2, articular facets of the capitular portions of the first and second ribs; I.V, interventral; N1, N2, N3, first, second, and third neural arch, formed by basidorsalia (B.D); Od, odontoid process = first centrum; Pz, posterior zygapophysis; R1, R2, ribs; Sp, detached spinous process of the first neural arch; t1, t2, tubercular attachments of the first and second ribs; 1, 2, 3, 4, "intercentra" = basiventrals; I, II, III, position of the exit of the first, second, and third spinal nerves.
As mentioned on p. [278] the vertebrae of the Reptilia and those of all other Amniota are gastrocentrous; that is to say the centra or bodies of the vertebrae are formed by the pairs of interventralia, while the basiventralia are reduced, persisting either as so-called intercentra or wedge-bones, or as intervertebral pads, or disappearing altogether. At the earlier stages of development the gastrocentrous vertebrae behave in the same way as that described on p. [12] (Fig. 1), except that the interdorsal elements are suppressed from the beginning. If the remaining three pairs of constituent elements of each vertebra (the basidorsalia, forming the neural arch; the interventralia, forming the body or centrum; and the basiventralia) remain separate, the vertebrae are called temnospondylous (τέμνω, I cut, σπόνδυλος, a vertebra). If the neural arches and the centra are suturally united or are fused with each other, the vertebrae are called stereospondylous (στερεός, solid). In many Amniota the atlas or first vertebra remains in a relatively primitive, embryonic condition, and is temnospondylous but for the usual modification that its centrum becomes attached to that of the second vertebra, and forms the odontoid process of the latter. The composition of gastrocentrous vertebrae (cf. p. [282]) is best illustrated by the first and second cervical vertebrae of the Crocodile (Fig. 56, 3, p. [283]).
Concerning geographical distribution, even a cursory study shows that the sub-classes have come into existence at very different geological periods, and have each followed their own lines of dispersal.
CHAPTER VIII
PROREPTILIA–PROSAURIA–THEROMORPHA
Sub-Class I.–PROREPTILIA.
Permian Temnospondylous Reptiles with well-developed limbs and girdles of the terrestrial type.