308 COMPARATIVE ZOOLOGY. 
sheath covers the jaws. The teeth are never fastened in 
sockets, except in Crocodiles. The jaws are usually very 
wide. The heart has three chambers, save in Crocodiles, 
where the ventricle is partitioned.“ But in all cases a 
mixture of arterial and venous blood is circulated. The 
lungs are large, and coarsely cellular. The limbs, when 
present, are provided with three or more fingers as well as 
toes. 
There are four orders of living Reptiles—the first two 
have horny scales, and two external nostrils; the others 
have bony plates combined with scales, and one external 
nostril : 
1. Ophidians, or Snakes, are characterized by the ab- 
sence of visible limbs ;“° by the great number of vertebree, 
amounting to over 400 in the great Serpents; by a corre- 
sponding number of ribs, but no sternum ; and immovable 
- SS » : " uM, I Va \ | 
= WSs AY Ny YUNA yo! 
Fie. 288.—Adder, or Viper (Vipera berus). England. 
transparent eyelids. The tongue differs from that of near- 
ly all other reptiles in being bifid and extensile. The 
mouth is very dilatable. The skin is frequently shed, and 
