98 PBACTICAL ZOOLOGY. 



Geology tells us, from the remains /ound in the rocks, 

 that most of the animals now living are different from 

 those of early ages. 



The earlier Batrachians were tailed; they were sala- 

 mander-like ; the frogs and toads appeared later. Thus 

 the different stages of development of the frog repeat the 

 order of appearance of Batrachia in geologic succession, 

 and review the classification of Batrachia from the lowest 

 to the highest. 



EXTERNAL FEATURES OF THE SNAKE. 



Examine the scales ; observe their relation to each other 

 and to the skin. A scale having a ridge running length- 

 wise in the middle line is carinated ; if there be no such 

 ridge, the scale is called smooth. How many rows of 

 scales are there, not counting the wide plates below? 



These wide plates along the belly, as far back as the 

 anus, are the gastrosteges ; count them. The plate im- 

 mediately in front of the anus is the anal plate; if it is 

 of one piece, it is called entire ; if of two pieces, it is bifid. 

 The plates under the tail are the urosteges ; how do they 

 differ from the gastrosteges? count them. Why should 

 there be these large plates below instead of the smaller 

 Icales found on the upper surface? How does a snake 

 crawl ? 



Keep a live snake, and watch its breathing movements. 

 Does the interval of rest occur after breathing in or after 

 breathing out ? Touch the eyeball with a stick ; can the 

 snake wink? Watch the movements of the tongue. 



