66 ELEMENTS OF COMPARATIVE ZOOLOGY. 



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 or firmly united to the skull; and the pre- 

 maxillary 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 

 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. In 

 the development of the eggs two structures, amnion and 

 allantois, are formed, which never occur in the Ichthy- 

 opsida. 



Reptiles are most abundant in the tropics, and are lack- 

 ing 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. 



