342 SYSTEMATIC ZOOLOGY. 



GRADE II. SAUROPSIDA. 



Although we naturally associate the birds with the 

 warm-blooded, hair-bearing mammals, they are structur- 

 ally far nearer the reptiles; hence the group which con- 

 tains the reptiles and birds is called Sauropsida, which 

 means lizard-like. The Sauropsida are distinguished from 

 the Ichthyopsida (p. 317) by the fact that at no stage of 

 development are functional gills present, and there is 

 never a metamorphosis. Scales, which are always present, 

 lie not between the two layers of the skin (see p. 318) but 

 are composed of the outer layer. The eggs are always 

 very large and in their development two structures, the 

 amnion and allantois, always occur. The sternum, when 

 present, is always- connected with the ribs. From the 

 mammals they are marked off by the absence of hair, the 

 position of the quadrate as the suspensor of the lower 

 jaw (p. 294), the articulation of the skull to the vertebral 

 column by a single condyle, by the large eggs, and by the 

 existence of a cloaca, a common tube into which the di- 

 gestive, excretory, and reproductive organs empty. There 

 are two classes of Sauropsida, Reptilia and Aves. 



CLASS I. REPTILIA (REPTILES). 



The living reptiles closely simulate the Batrachia, and 

 in fact the frogs, toads, and salamanders are reptiles in 

 popular parlance. The short-bodied turtles are paralleled 

 by the frogs, the lizards by the salamanders, and the 

 snakes by the blindworms. Yet the differences between 

 the two groups are many and important. 



