xin 



PHYLUM CHORDATA 



331 



The limbs are fully developed though short. In some (land and 

 fresh-water Tortoises) they are provided each with five free digits 

 terminating in curved horny claws ; in the Turtles the digits are 

 closely united together, and the limb assumes the character of 

 a " flipper " or swimming paddle. The cloacal aperture is longi- 

 tudinal. 



The Crocodilia, the largest of living Reptiles, have the trunk 

 elongated and somewhat depressed, so that its breadth is much 

 greater than its height. The snout is prolonged, the neck short, 

 the tail longer than the body and compressed laterally. The 

 limbs are relatively short and powerful, with five digits in the 

 manus and four in the pes, those of the latter being partly or 

 completely united by webs of skin. The eyes are very small ; 



Fia. 991. Grecian Tortoise (Testttdo grceca). (After Brehm.) 



the nostrils placed close to the end of the snout and capable of 

 being closed by a sphincter muscle. The cloacal aperture is a 

 longitudinal slit. The dorsal and ventral surfaces are covered 

 with thick, squarish horny scales, often pitted or ridged, those 

 of the dorsal surface of the tail developed into a longitudinal 

 crest. 



Integument and Exoskeleton. Characteristic of the Squa- 

 mata is the development of horny plates which cover the entire 

 surface, overlapping one another in an imbricating manner. These 

 differ considerably in form and arrangement in different groups ; 

 sometimes they are smooth, sometimes sculptured or keeled. 

 Sometimes they are similar in character over all parts of the 

 surface ; usually there are specially developed scales the head 

 shields covering the upper surface of the head. In the majority 

 of Snakes the ventral surface is covered with a row of large 

 transversely elongated scales, the ventral shields. In certain Lizards 



