REPTILES. 345 



is lacking in most snakes, being represented by two bones 

 in the boas. The feet, when present, are usually of the 

 normal type, the bones of the forearm (ulna and radius) 

 and of the shank (tibia and fibula). being separate, and 

 the toes, five in number, provided with claws. 



In the embryo, gill-slits are partially developed, but no 

 functional gills occur. The lungs are well developed; the 

 left one being reduced or absent in the snakes and snake- 

 like lizards. Respiration is effected by means of the ribs, 

 except in the turtles, and there by a special muscle. 



Both ovaries are developed. The eggs are large, and, in 

 those reptiles which lay eggs, are covered with a limy shell. 

 A few snakes and lizards bring forth living young. 



Reptiles are most abundant in the tropics, and are lacking 

 in cold regions. They are mostly flesh-eaters, some living 

 on insects, others on larger forms. Some live on land, 

 some in fresh water, and some in the sea. All living 

 forms can be arranged in four orders, Lacertilia, Ophidia, 

 Testudinata, and Crocodilia. 



ORDER I. LACERTILIA (Lizards). 



In these the quadrate bone is movable, but the under 

 jaw cannot be displaced (cf. Snakes). Legs are usually 

 present, but either or both pairs may disappear. When 

 the legs are absent the body is exceedingly snake-like, 

 but these forms, like all other lizards, may be distin- 

 guished at once from the true snakes by the presence of 

 small scales on the belly. Only one lizard, the 'Gila 

 monster 7 of Arizona, has the reputation of being poison- 

 ous, but in former times many, like the basilisk, were 

 fabled to have most deadly powers. Among the more 

 interesting forms are the 'glass snakes/ so called from 



