_The Axis._ The centrum differs from those following it (described above) mainly in its close articulation (not fusion) with the large odontoid process; this process not only projects into the atlas, as is usually the case, but articulates with its postero-lateral border on each side, and is distinctly visible in a lateral view of the neck (Fig. 17, 3). Like the rest of the cervical vertebræ the posterior surface of the centrum is convex. The neural arch of the atlas differs from those following mainly in having a much wider (in an antero-posterior direction) neural spine. The lateral processes and those bearing the prezygapophyses are also less strongly developed than on the following vertebræ. [Illustration: FIG. 18. ANTERIOR VIEW OF A, A LATE THORACIC AND B, THE FIRST SACRAL VERTEBRA OF A YOUNG CROCODILE (_C. palustris_). × ¹⁄₃. (After Reynolds.) 1. neural spine. 2. process bearing prezygapophysis. 3. facet for articulation with the capitulum of the rib. 4. sacral rib. 5. surface which is united with the ilium. 6. concave anterior face of centrum.] _The Thoracic Vertebræ._ The first thoracic vertebra differs scarcely at all from the ninth cervical; and the tenth thoracic differs from the first lumbar only in bearing a short rib. Only the first three thoracic centra bear the hypapophyses noted in connection with the cervical vertebræ. The ribs of the first two thoracic vertebræ articulate with them by two processes, as in the typical cervical vertebræ; the other ribs articulate only with the transverse process. The fourth thoracic may be described as a type of this region (Fig. 18, A). Its centrum is rather longer than in the first two thoracic and in the cervical vertebræ and has no process for articulation with the head of the rib, otherwise it is essentially the same. Like all of the vertebræ behind it and unlike those in front it is apparently completely fused with its neural arch. The neural arch is very broad (in an antero-posterior direction) and is extended dorsally as a wide neural spine (1). The neural spines of the following thoracic and the first two or three lumbar vertebræ are increasingly broad and truncated. The transverse processes are very broad, long and thin, and in the third to eighth vertebræ they have two articular surfaces, an anterior and more medial one for articulation with the head of the rib (3) and a posterior and more distal one for articulation with the tubercle of the rib. These two surfaces approach each other as the vertebræ are followed caudad until, in the last two thoracic vertebræ, they form practically one surface. The processes of the pre- and postzygapophyses spring from the arch at the base of the transverse process; the former surface is directed dorsally and medially, the latter ventrally and laterally. The intervertebral foramina are smaller and more nearly circular than in the cervical region, and are more closely surrounded by bone.