506 



ZOOLOGY. 



nine in number. The jaws are toothless, being, as in birds, 

 encased in horny beaks ; there are rarely fleshy lips ; the 

 tongue is spoon-shaped and immovable. The heart consists 

 of two auricles and a ventricle. The brain has larger cere- 

 bral lobes than in the lizards. The eyes have a third lid, or 

 nictitating membrane. The student can best obtain an 

 idea of the organization of the turtles by studying the skel- 

 eton and dissecting a turtle with the aid of the accompany- 

 ing description and figure of the common turtle.* 



The common swamp-turtle (Chrysemys picta) is a good 

 type of the Chelonia. The animal is enclosed in a hard shell 

 made up of an arched dorsal portion, and a flat ventral por- 



Fig. 445. Ventral epidermal plates of Chrysemys picta. Drawn by C. S. Minot. 



tion, the two connected laterally, but widely separated an- 

 teriorly to give exit to the head and fore limbs, and pos- 

 teriorly for the tail and hind limbs. These parts can all be 

 withdrawn within the protecting shell, by being doubled or 

 folded back upon themselves. The soft parts of the skin are 

 covered with scales, formed by overlapping folds. The limbs 

 are stout ; upon the anterior feet there are five, upon the 

 posterior four claws. On the under surface of the short 

 tapering tail near its base is the wide opening of the cloaca. 

 The ventral plastron consists of twelve symmetrical pieces, 

 six on each side. Fig. 445. The first and last pair are tri- 

 angular, the others are four-sided ; the fourth pair is the 



*This description has been prepared and the illustrations drawn by 

 Dr. C. S. Minot. 



