344 



SYSTEMATIC ZOOLOGY. 



The vertebrae are usually proccelous, and the vertebral 

 column is divisible into the regions of neck (ribless). 

 thorax (with ribs), lumbar (ribless), sacrum (usually two 

 vertebrae which connect with the pelvis), and tail; but in 

 snakes these distinctions fail, and only trunk and tail ver- 

 tebrae are recognizable. A breast-bone is present in 



FIG. 142. Brain of Snake, c, cerebrum; d, cerebellum; o, optic lobes; I, 



olfactory nerve; II, optic nerve. 



lizards and alligatois, but none occurs in turtles or snakes. 

 The skull articulates with the vertebral centrum by a single 

 surface (condyle). The hinder angle of the lower jaw is 

 connected with the skull by the quadrate bone, which may 

 be free (fig. 143), or firmly united to the skull; and the 



FIG. 143. Skull of Garter-snake (Eutoenia sirtalis}, showing the attachment 

 of the- lower jaw to the skull by means of the quadrate bone, q. (Slightly 

 enlarged.) 



premaxillary and maxillary bones are firmly united to the 

 rest of the skull. Teeth are usually present, and in the 

 alligators these are inserted in sockets. The shoulder- 

 girdle (lacking in snakes) is much like that of frogs, the 

 clavicle, however, being absent in alligators. The pelvis 



