THE OPHIDIA. 



201 



All the Snakes possess a scaly epidermic investment, which 

 it usually shed in one piece, and reproduced at definite inter- 

 vals. As a general rule these scales are flat, and overlap one 

 another ; but sometimes, as in Acrochordiis^ they become 

 more tubercle-like, and do not overlap. In the Rattlesnakes 

 (Grotalus) the body is terminated by several loosely- conjoined 

 rings o£ horny matter, which consist of the modified epidermis 

 of the end of the tail. 



The derm does not become ossified in the OpJiidia, 



The number of the vertebra in the Snakes is always con- 

 isiderable, and in some cases becomes very great, amounting 

 to more than four hundred in some of the large Pythons. The 

 spinal column is divisible only into caudal and precaudal re- 

 gions, as there is no sacrum, nor any distinction between cer- 

 vical, dorsal, and lumbar vertebras. The atlas and the odon- 

 toid vertebra are similar to those of the Lizards, and the atlas 

 is the only precaudal vertebra which is devoid of ribs. The 

 centra have nearly hemispherical articular surfaces, and thus 

 differ from those of ordinary LacertiUa^ while the superadded 

 articular processes found only in certain Lizards attain a great 

 development in the Snakes. The zygapophyses are broad 

 and flattened, and the outer surfaces of the anterior pair are 

 commonly prolonged into a process. The anterior surface of 



Tt.S. 



FiQ. 71. — ^Anterior and posterior views of the dorsal vertebra of a Python : z. s., zygosphene ; 

 s. a., zygautrum ; jj. 3., prezzgapophyses ; pt. s., postgygapophyses ; t. p., transverse 

 processes. 



the arch above the neural canal is produced into a strong 

 wedge-shaped zygosphene, which fits into a corresponding 

 zygantrum of the next preceding vertebra ; and, on the pos- 

 terior surface of the arch, there is a zygantrum for the zygo- 

 sphene of the next preceding vertebra. (Fig. 71.) 



