102 REPTILES AND BATRACHIANS 



into five groups, which pass into one another through 

 intermediate forms. They are — 



1. Burrowing Snakes, which live underground, feeding 

 on earthworms and insects. They have usually a very 

 small head, which is not distinct from the neck, a small 

 mouth, not very expansible, a short tail, and a cylindrical 

 body. They are mostly harmless. 



2. Ground Snakes, which live above ground and seldom 

 climb or enter water. They are the typical snakes, repre- 

 sented by poisonous as well as non-poisonous species. 



3. Tree Snakes, expert climbers, spending most of their 

 life in bushes or trees. They may as a rule be recognized 

 by their slender form, their long and sometimes prehensile 

 tail, and by the ventral shields being frequently angulate 

 on the sides. As in the last group they may be poisonous 

 or harmless. Their food consists mainly of birds, lizards, 

 and tree-frogs. 



4. Fresh-water Snakes, which have the nostrils situated 

 on the top of the snout. They feed almost entirely on 

 frogs and fish. All are more or less harmless to man. 



5. Sea Snakes, which, in addition to having superior 

 nostrils, have strongly compressed, rudder-shaped tails. 

 They are all highly poisonous. 



Some tree-snakes, Chrysopcleea in particular, are known 

 as " flying-snakes," from their habit of parachuting from 

 tree to tree, the body being kept quite rigid during flight. 

 Shelford, who has investigated this behaviour, states that 

 the ventral surface between the lateral keels, which act as 

 hinges, can be drawn inwards, so that the snake becomes 

 deeply concave along the ventral surface ; at the same time 

 there is a flattening of the body. During this muscular 



