250 BRA^X'H CHORD ATA 



dermal bones, all fused into one bony carapace which is covered 

 by epidermal plates. The plastron consists of epidermal plates 

 underlaid with bony, shields. The head, limbs, and tail may be 

 more or less withdrawn under the carapace for protection; in 

 some forms so completely that there is no part of the turtle 

 visible outside of the shell. 



They have four short, strong, clawed, pentadactyle limbs, or, 

 in the case of some marine forms, flippers. The marine paddle- 

 limbed forms are usually designated as turtles, and those with 

 walking limbs as tortoises. 



The hones of the head are firmly united. The jaws are 

 toothless, but are inclosed in sharp horny beaks, with which 

 they seize and crush their prey and then swallow it whole. 

 The pectoral arch is a "triradiate structure, of which the 



Fig. 206. — Skeleton of snapping-turtle with portion of carapace sawed oflf 

 to show interior. Cleaned and mounted by students ; (much reduced.) 



most ventral and posterior ray ending in a free extremity is 

 the coracoid, while the other two are the procoracoid and the 

 scapula with the suprascapula, which are fused at their glenoid 

 ends."^ The pelvis is strong, consisting on each side of the 

 pubis, the ischium, and the ilium. These meet at the articular 

 surface of the hind limb. The vertebrae are few as compared 

 with those of the snake. Those of the neck fit into one another 

 with ball-and-socket-joint. There is no trace of a sternum or of 

 sternal ribs. 



Digestive System. — The tongue is usually soft and wide and 

 not extensible. The esophagus is covered with " conical pro- 

 jections pointing toward the stomach." There is no cecum. 



Food. — Some species are carnivorous, some are herl^ivorous, 



1 Parker and Haswell, vol. ii, p. 329. 



