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 vertebne are always fully ossified. Only 

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

 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 centrum 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 vertebrae of the cer- 

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

 The paired bones of the inferior arches (chevron 

 bones) 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 the centra of the anterior cervical vertebrae. In Chamaeleons, 

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

 <(ll(ts (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 zygotyhcncs and zyyantra (Fig. 982). The zygosphene 



FKJ. 981. Vertebra of 

 Sphenodon, show- 

 ing the amphiccelous 

 centrum (,). (After 

 Headley.) 



Jit.*. 



Kic. !>S2. Vertebra of Python, anterior and posterior views, n. s. neural spine; p. z. pre- 

 zygapophyses ; { >t. z. post-zygapophysis ; t. p. trans verse processes ; 2. a. zygantrum ; z.s. zygo- 

 sphuiie. (After Huxley.) 



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

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

 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 



