The vertebral column is very primitive. The atlas is still typically temnospondylous. The first intercentrum or fused pair of basiventrals is broad and thick, and forms the ventral half of the atlas-ring, which articulates with the first centrum and with the second intercentrum. The irregularly shaped neural arches remain separate from each other and from the centrum; they carry on the dorsal side a pair of disconnected supradorsals, the so-called pro-atlas. The second intercentrum is fused with the first and second centrum. The second to ninth intercentra have low median ridges or knobs, and are as a rule more firmly attached to the cranial ends of the centra. Those of the trunk are small. From the third or fourth caudal vertebra {297}backwards they appear as chevrons, articulating more with the vertebra in front than with the one behind. The bases of the right and left chevrons are frequently fused across so that the caudal canal is completely surrounded by bone, a feature common in Dinosaurs. Every intercentrum, be it a pair of chevrons, or an unpaired nodule, or crescent, extends dorsalwards into a fibro-cartilaginous ring which surrounds the chorda. The centra of the vertebrae are deeply amphicoelous, the cavity being filled throughout life by the chorda; but the middle of the centra is solid. Most of the caudal vertebrae are transversely divided into two parts, the posterior of which carries the greater share of the arches; they resemble in this respect those of lizards, and the lost tail is likewise reproduced. The first three ribs are represented by bands of connective tissue. The first is attached to the side of the first intercentrum; the second arises from the second intercentrum, and forms a small tubercle on the side of the second centrum; the third behaves similarly. The vertebral arteries and lateral strands of the sympathetic nerve-chain pass through these double basal attachments of the reduced ribs. The other ribs are osseous; they possess short capitula which retain their partly intercentral attachment, while the short tubercula are carried by low processes of the centra, not of the neural arches. Already in the thoracic region both capitulum and tuberculum merge into one facet, at first dumb-bell shaped, further towards the tail oval, gradually shifted backwards and dorsalwards upon the middle of the centrum, until the facet reaches and ultimately lies right across the neuro-central suture. The first few caudal vertebrae also possess ribs, which are however very short and fuse with the diapophyses, immediately below which lies the neuro-central suture. [Illustration: FIG. 60.–The first three cervical vertebrae of _Sphenodon_. 1, 2, 3, 4, Intercentra; _C_{1}-C_{3}_, centra; _N_{1}-N_{3}_ neural arches.]