m PHYLUM CHORD AT A 319 



of the Cephalopoda. Less conspicuous and rapid changes of colour 

 .take place in Anguis and in some Snakes. 



In the Chelonia, scales, when developed, are confined to the 

 head and neck, the limbs and the tail, but in all of them, with the 

 exception of the Soft Tortoises, both dorsal and ventral surfaces 

 .are covered by a system of large horny plates. A series of horny 

 head-shields usually cover the dorsal surface of the head. Beneath 

 the horny plates of the dorsal and ventral surfaces are the bony 

 carapace and plastron, partly composed of dermal bones, but so 

 intimately united with elements derived from the endoskeleton 

 that the entire structure is best described in connection' with the 

 latter (vide infra}. 



In the Crocodilia, the dorsal surface is covered with longitudinal 

 rows of sculptured horny plates, beneath which are bony dermal 

 .scutes of corresponding form. The ventral surface of the body is 

 covered with scales like those of a Lizard. 



A periodical ecdysis or casting and renewal of the outer layers 

 <of the horny epidermis takes place in all the Reptilia. Some- 

 times this takes place in a fragmentary manner ; but in 

 Snakes and many Lizards the whole comes away as a continuous 

 slough. 



Endoskeleton. The vertebras are always fully ossified. Only 

 in the Geckos and Hatteria (Fig. 925) are the centra amphi- 

 ccelous 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 pro- 

 jecting into a cup-like concavity on the an- 

 terior face of the next. In Hatteria and the 

 Geckos a series of wedge-shaped discs (inter- 

 centra) are intercalated between the vertebras 

 of the cervical, part of the thoracic, and caudal 

 regions. The paired bones of the inferior 

 arches (chevron bones) are attached to these FIG. 025. vertebra of 



T 1 T . Hatteria, snowing 



bones when they are present. In the Lizards the amphicreious 

 in general and the Crocodiles there are inferior Headiey.) 

 processes (hypapophyses), perhaps representing 

 intercentra, situated below the centra of the anterior cervical 

 vertebrae. In Chameleons, Hatteria, and the Crocodiles there is a 

 median bone, the pro-atlas, 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 zygosphenes and zygantra (Fig. 926). The zygosphene 

 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 



