THE SNAKES 201 



canal is perfectly formed and the anterior surface of the 

 poison-conducting teeth shows but a faint suggestion of 

 a groove. It must not be taken for granted, however, 

 that these "intergradations" show the three groups to 

 have originated from a common source. The evolution 

 of the Elapine and the Viperine snakes has sprung from 

 quite distinct branches. The trend of evolution seems 

 to point to the greater number of snakes becoming 

 poisonous, and the elaboration of the fang mechanism 

 among the forms now poisonous, even with the Vipers, 

 if that be possible. 



On account of the great array of serpent life and the 

 peculiar external similarity of the smaller species in 

 scale formation and arrangement of the head plates, 

 technical writers have found it necessary to employ the 

 dentition as the prime factor in classifying the ophidians. 

 But in connection with the dentition, the rows of scales 

 are counted, the structure of the head shields noted and 

 the number of abdominal plates carefully taken. For 

 the benefit of the beginner it is well to state that snakes 

 have either keeled (carinated) or smooth scales. Ex- 

 amples of the former are the Water Snakes (Tropido- 

 notus) and of the latter, the North American Racers 

 (Zamenis). 



The movements of most serpents are singularly quick 

 and graceful. They depend largely upon the broad 

 plates of the abdomen, the sharp, overlapping posterior 

 edges of which offer a substantial purchase. In crawl- 

 ing slowly, a snake really walks upon these plates by 

 bringing them forward in series along the body, then 

 pulling them back again. Working in this fashion a 

 serpent may progress in a perfectly straight line; a 

 moment later it may be frightened, when it threads its 

 way among twigs and stones, gliding swiftly in a series 



