Page:EB1911 - Volume 23.djvu/171

Rh and fuses with, the second intercentral piece. The entire atlas remains in a primitive, typically temnospondylous condition. On the other hand, in some Pleurodira, e.g. Platemys and Chelys, all the constituent parts of the atlas coössify and form a complete, solid vertebra, which articulates by a concave-convex joint with the true centrum of the second vertebra. The normal number of cervical vertebrae is eight in all Chelonians. The last cervical has sometimes, e.g. Chelydra, a very peculiar shape with strangely modified articular facets, in correlation with the retractile neck. The neural spines of the trunk vertebrae broaden out and fuse with the neural plates of the carapace. A tertiary modification takes place in many Pleurodira with the reduction of the neurals by the costal plates, which then meet in the dorsal line and cover the neural spinal processes. The caudal vertebrae are often much reduced in size, although not always in numbers, when the tail is very short, as in the marine turtles. In various species of Testudo about half a dozen of the last caudal vertebrae fuse together into a veritable urostyle, which is covered with a claw- or nail-shaped sheath of horn. In some of the gigantic tortoises of Mauritius this caudal vertebral complex is fully 3 in. long and 2 in. broad, of an extraordinary appearance.

The vertebrae of the Lacertae, or Lizards proper, are a direct further development of those of Sphenodon. The chorda disappears; the vertebrae are procoelous, with an articulating knob behind. Intercentrals, in the shape of osseous, unpaired nodules or wedges, persist on most of the cervical vertebrae; they are absent in the trunk and reappear in the tail, either as wedges or with chevrons. The first intercentral forms the central half of the atlas, with the neural half of which it is connected by suture. The second fuses mostly with the cranial end of the second centre and with the caudal and ventral surface of the odontoid, forming a downward-directed hook. Frequently the fusion remains incomplete, or the wedges may completely merge into the epistropheal mass without leaving any outward traces. Boulenger has made the important observation that the intercentra of the tail are sometimes paired, e.g. in Heloderma. When the caudal vertebrae are strongly procoelous, the knob is very long and the chevrons are attached to its neck, having shifted on to the vertebra in front, while their basal intercentral piece, or pieces, remain in the original position. In Ophisaurus the chevrons are absolutely fused with the caudal ends of the centra and thus assume a superficial resemblance to the vertebrae of Urodela. The splitting of the tail-vertebrae and regeneration have been described on a previous page. The trunk-vertebrae of the Tejidae and the larger Iguanidae possess additional articulating processes and facets, besides the usual processes. The Zygosphene is a wedge-shaped process with two articular facets, which projects forward from the anterior side of each neural arch. The Zygantrum forms a corresponding excavation with a pair of articular surfaces on the hinder side of the arch. The crests on the tail and trunk of many lizards, e.g. Iguanidae, are entirely tegumentary structures and not supported by the axial skeleton, except in some chameleons, e.g. ''Ch. cristatus, and in the peculiar genus Brookesia''; in these the accessory much-complicated processes are enormously elongated and support the high cutaneous crest which arises from the back, especially in B. ebenaud.

27.—Lateral aspect of two trunk vertebrae of Python. a, articular processes of the zygapophyses; na, neural arches; ns, neural spines; t, parapophyses; zs, zygosphene. The vertebrae of the snakes are procoelous (figs. 27, 28, 29). Besides the zygapophyses, they have zygosphenes on the neural arches; the ribs articulate with the parapophyses. Long, unpaired hypapophyses arise from the centre of the anterior neck and trunk vertebrae to a variable extent. In Dasypeltis and Rhachiodon a considerable number of these processes perforate the oesophagus and act as crushers of the shell of the eggs which these snakes swallow. The often-repeated statement that these processes are capped with enamel is erroneous. The caudal vertebrae are devoid of chevron bones, but they carry paired hypapophyses, and they have transverse processes which also are generally bent downwards.

28.—Posterior aspect of a trunk vertebra of Python (from nature). a, zygapophses; b, ball on the surface of the centrum; t, parapophysis; zg, zygantrum. Lastly, the numbers of vertebrae composing the whole column and its various regions. In the snakes we can distinguish only

between atlas and epistropheus, trunk and tail. The numbers vary exceedingly, in the trunk up to several hundred.

29.—Anterior aspect of a trunk vertebra of Python (from nature). a, zygapophyses; c, cup on the surface of the centrum. The tail may contain only a few, e.g. in the burrowing Typhlops, Glauconia, Uropeltis; or it may be very long, as for instance in Boa. There is no obvious reciprocal correlation between the length of the trunk and the tail. In the other orders of reptiles the neck is well marked, except in the snake-shaped lizards. If we define as first thoracic vertebra that which is the first connected with the sternum, all those anterior being cervical, the neck-vertebrae number 5 in chameleons, 7 in Sphenodon, 8 in the Chelonians and in the lizards, with the exception of the majority of Varanus, which have 9 like the Crocodilia.

The ribs, having arisen as lateral, separated off processes from the basiventral elements, show many modifications in their proximal attachments. These can be best studied on the skeleton of a young crocodile (fig. 25, 7 and 8). The first pair of ribs is very long and broad, attached to the unpaired ventral piece of the atlas-ring; the tubercular portion is indicated by a very small rugosity. The second pair of ribs is still larger; the capitulum attached to the second intercentral piece which fuses with the odontoid process; the tubercular process is weak or represented only by a ligamentous connexion with a small knob of the odontoid process; consequently the tuberculum has shifted its attachment away from the second vertebra. The other cervical, and the anterior thoracic, ribs have complete