2 INTRODUCTION 



prey much exceeding their own calibre ; hence the 

 bones of the skull, on which a prehensile function 

 devolves, are loosely attached to the cranium by 

 ligamentous elastic tissue, or articulated in such a 

 manner as to permit a wide buccal expansion ; whilst 

 the absence of a sternum and the mobile attach- 

 ment of the ribs allow a corresponding dilatation of 

 the body as the prey descends into the digestive canal. 



The fatal venom which many of these Reptiles 

 possess has so impressed the mind of men, even the 

 scientific, that for a long time snakes were primarily 

 divided into poisonous and non-poisonous, a classi- 

 fication in which the more important characters, 

 derived from the general structure, and especially 

 from the skull, were subordinated to the physio- 

 logical. Such a system was far from reflecting 

 natural relationships. Besides, as our knowledge 

 progressed, drawing a distinction between poisonous 

 and harmless snakes became more and more diffi- 

 cult, so many snakes previously regarded as harmless 

 proving to be poisonous in various degrees — at least 

 enough to paralyze the small prey on which they 

 subsist, if not to be of serious danger to man. 



In the division into families, as followed in this 

 work, the presence or absence of a poison organ is 

 left out of consideration. Further, in this as in 

 many other groups of the animal kingdom, external 

 characters do not furnish trustworthy indications 

 for higher divisions, and the definitions of the 



