154 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



developed ribs. They are confined to the tropics, and 

 are subterranean in habit. 



CLASS IV. Reptilia 



These air-breathing, cold-blooded vertebrates are dis- 

 tinguished from all fishes and amphibians by never hav- 

 ing gills, and from birds by being covered with horny 

 scales or bony plates. The skeleton is never cartilag- 

 inous (Fig. 310); and the skull has one occipital condyle. 

 The vertebrae are ordinarily concave in front; and the 

 ribs are well developed. With few exceptions, all are 

 carnivorous ; and teeth are generally present (Figs. 231, 

 235), except in the turtles, where a horny sheath covers 

 the jaws. The teeth are never fastened in sockets, ex- 

 cept in crocodiles (Fig. 224). The jaws are usually very 

 wide. The heart has three chambers (Fig. 273), save in 

 crocodiles, where the ventricle is partially partitioned. 

 But in all cases a mixture of arterial and venous blood is 

 circulated. The lungs are large, and coarsely cellular 

 (Fig. 281). The limbs, when present, are provided with 

 three or more fingers as well as toes. 



There are about three thousand species of living rep- 

 tiles, and of these there are three main orders : the first 

 has horny scales, the others have bony plates combined 

 with scales. 



i. Squamata, including the lizards and the snakes. 

 The lizards (Lacertilia) may be likened to snakes provided 

 with fourlimbs, each having five digits. 37 The body is cov- 

 ered with horny scales. All have teeth, which are simple 

 in structure; and the halves of the lower jaw are firmly 

 united in front, while those of snakes are loosely tied 

 together by ligaments. Nearly all have a breastbone, 

 and the eyes (save in the gecko) are furnished with 

 movable lids. In the common lizards and chameleon, 

 the tongue is extensile. The tail is usually long, and in 



