ADAPTATIONS 107 



the limbs for a gliding mode of motion. The sea-snakes have 

 been further modified for a pelagic existence by the lateral com- 

 pression of either the tail alone or the whole body and tail, and 

 the reduction in the size of the scales on the under surface, which 

 are frequently not larger than those on the upper parts. 



Sea-snakes, of which there are several genera, such as 

 Hydrophis and Enhydrina, with somewhere about fifty species, 

 are mainly pelagic, most of them dying if kept for any length 

 of time out of water ; they even breed at sea, and for this pur- 

 pose are all viviparous. The largest species attain a length of 

 about six feet. All are highly poisonous, and feed upon fishes 

 which they kill by means of their poison-fangs. A single 

 species, Distira semperi, is found in the land-locked fresh-water 

 Lake Taal in the island of Luzon in the Philippines ; this 

 species, presenting an exact analogy with the seals which in- 

 habit Lake Baikal, in Central Asia. The marine forms are 

 confined to the tropical seas, in which they are met with from 

 the Persian Gulf to Central America. In Samoa they are eaten 

 by the natives. 



It should be mentioned that, in addition to the true sea- 

 snakes, a species of a very different group of serpents, namely 

 Hypsirhina Jiydrinns y of Siam and the Malay Peninsula, has 

 taken to a marine existence, and has consequently acquired a 

 form of body recalling that of the HydropJiiina. It has much 

 the same habits as the members of the latter, swimming far out 

 to sea in search of the fishes which form its prey. Here we 

 have an instance of the assumption of the bodily form and 

 habits distinctive of a particular group by a member of a totally 

 different section. 



The marine turtles, of which there are two distinct families, 

 namely the Chelonida? represented by the true turtles, such as 

 the green turtle (Chelone my das) and the logger-head (Thalas- 

 sodielys caretta), and the Dermochelyidce, which includes only the 

 luth or leathery turtle {Dermochelys coriacea) afford a still more 

 marked instance of a double adaptive modification. The true 

 turtles are undoubtedly descended from terrestrial chelonians, 

 which have themselves been specially modified by the develop- 

 ment of the characteristic shell from some unknown type of 

 more ordinary reptiles ; the intermediate type between turtles 

 and tortoises being represented by certain extinct Jurassic 



