ii2 REPTILES 



existence, with the assumption of a compressed form of body 

 or tail, and the loss of the enlarged scales on the under surface, 

 which would manifestly be an inconvenience in swimming. 

 These large ventral scales have also completely or partially dis- 

 appeared in four families of which the members lived a wholly 

 or partially subterranean life ; the disappearance being corre- 

 lated with completely burrowing habits. These four families 

 are the blind snakes, or TypJdopida, the ground-snakes, or 

 Glaucomidce, the coral-snakes, or Ilysiidce, and the shield-tails 

 or Uropeltidcc ; the latter taking their name from the oblique 

 truncation of the tail, the shield-like terminal surface of which is 

 said to perform the work of excavating the burrow. These 

 various burrowing snakes are evidently degraded types which 

 took to a burrowing mode of life at an early stage of the evolu- 

 tion of the Ophidia ; this early branching-off from the main 

 stock being demonstrated by the retention in some of them of 

 rudiments of the pelvis. Probably the four families have been 

 modified for the same mode of existence independently of one 

 another. The Typhlopidce have at the present day a wide geo- 

 graphical distribution, although they are unknown in New 

 Zealand, and at one time were probably cosmopolitan. Al- 

 though they are called blind snakes, they have not really lost 

 their eyes, which are, however, hidden by folds of skin. They 

 are confined to tropical countries, and feed largely upon earth- 

 worms. 



In these burrowing snakes the head, body, and tail are 

 generally cylindrical, in adaptation to their particular mode of 

 life ; and in many of the more typical snakes, such as pythons 

 and the grass-snakes, there is a marked tendency towards this 

 cylindrical form in the body, although its under surface is more 

 or less flattened for the purpose of gliding on the ground. In 

 certain snakes dwelling in desert sands, more especially many 

 members of the viper group, such as the puff-adder {Bids arie- 

 tans) and the horned viper (Cerastes comutiis), the head and 

 body are much depressed and flattened, the head being re- 

 markably broad and flat. This depressed form is obviously 

 for the purpose of enabling these reptiles to lie close on the 

 sand, and thus the more easily to escape detection ; and 

 affords an exact parallel to the acquisition of a similar de- 

 pressed type by sand-haunting lizards like Phrynosoma and 



