DEFINITION AND CLASSIFICATION 3 



families are therefore based exclusively on osteo- 

 logical characters. For those who wish to name 

 snakes with facility, the key which concludes the 

 chapter on External Characters will, however, remedy 

 this defect, and suffice for the identification of all 

 the European species without any reference to their 

 anatomy. Many attempts have been made to 

 furnish an easy criterion for the distinction of harm- 

 less from poisonous snakes, but the characters 

 hitherto suggested with this object can only be ap- 

 plied successfully to the small number of representa- 

 tives in a limited area. Thus, in Southern Australia 

 it might be stated that all snakes showing the regular 

 nine large shields on the upper surface of the head 

 are dangerous to man, whilst those with small 

 shields or scales are harmless ; but in most parts of 

 Europe this criterion would have to be reversed. In 

 some countries the shape of the pupil might be used 

 for the purpose, in others the size of the ventral 

 shields, or the presence or absence of a loreal shield, 

 between the nasal and the preocular, and so on. 

 But when we have to deal with the snakes of the 

 whole world, about 2,000 species, of which nearly 

 one-third are poisonous to a greater or less degree, 

 every attempt at a definition of the two categories 

 without regard to the dentition breaks down. Only 

 those who have made a study of the snakes of the 

 world can make a guess from the general appearance 

 as to an unknown form being poisonous or not, and 



