XIII 



PHYLUM CHORDATa. 



343 



the Exception of the Crocodiles. Sometimes this occurs in a 

 fragmentary manner ; but in Snakes and many Lizards the whole 

 comes away as a continuous slough. 



Endoskeleton. — The vertebrse are always fully ossified. Only 

 in the Geckos and Sphenodon (Fig. 981) are the centra amphicoslous 

 with remnants of the notochord in the inter- 

 central spaces, In most of the others the 

 centra are proccelous, a ball-like convexity on 

 the posterior surface of each centruin projecting 

 into a, cup-like concavity on the anterior face 

 of the next. In Sphenodon and the Geckos a 

 series of wedge-shaped discs (intercentra) are 

 intercalated between the vertebrse of the cer- 

 vical, part of the thoracic, and the caudal regions. 

 The paired bones of the inferior arches {chevron 

 lones) are attached to these bones when they 

 are present. In the Lizards in general and the 

 Crocodiles, there are inferior processes {hypapo- 

 physes), perhaps representing intercentra, situated 

 below tlie centra of the anterior cervical vertebrae 

 Sphenodon, and the Crocodiles there is a median bone, the pro- 

 atlas (Fig. 985, 0), intercalated between the atlas and the occipital 

 region of the skull. 



In . the Snakes and in Iguanas, in addition to the ordinary 

 articulating processes or zygapophyses, there are peculiar articular 

 surfaces termed zygosphcnes and iygantra (Fig. 982). The zygosphene 



Fig. 981.— Vertebra o£ 

 Sphenodon, show- 

 ing the amphicrelous 

 centrum ((?.). (After 

 Headley.) 



In Chamseleons, 



Fig. 982. — Vertebra of Python, anterior and posterior views, n. s. 

 zygapophyses ; pi. z. post-zygapophysis ; L p. transverse processes ; z 

 sphene. (After Hnxley.) 



neurdl spine ; p. z. pre- 

 a. zygantrum ; z.s. zygo- 



is a wedge-like process projecting forwards from the anterior face 

 of the neural arch of the vertebra, and fitting, when the vertebrae 

 are in their natural positions, into a depression of corresponding 

 form — the zygantrum — on the posterior face of the neural arch 

 of the vertebra in front. To this arrangement, as well as to the 



Y 2 



