262 



THE INDIAN SILLAGO. 



As the different kind of scales will necessarily be occasionally mentioned while 

 treating of the fishes, it will be useful to give a brief description of the different forms 

 of these appendages. 



THE scales are deposited by the action of the skin to which they adhere, forming 

 part of the structure called the external skeleton, and, according to the best compara- 

 tive anatomists, being closely analogous to the projecting spines that arm the body of 

 many fishes, to the fin rays, and even to the teeth. The beautiful series of transitions 

 through which anatomists have learned to ascertain the identity of the external skeleton 

 with the fin rays, the spines, and the teeth, extends over a vast number of species, 

 and is altogether too abstruse a subject to be admitted into these pages, in spite of 

 its exceeding interest. 



INDIAN SILLAOO. S///*o domlaa. 



The reader, however, who wishes to see a specimen of transition from scales to teeth, 

 may take one of the skate tribe say the eagle-ray and carefully examine the external 

 surface of the body and interior of the mouth. On some parts of the body the scales 

 are small, hard, and wart-like, while on others they are armed with projecting points. 

 Following the scales over the head as far as the lips, they will be seen assuming a 

 different shape, being there small, flattened, and aggregated together, forming a kind of 

 bony tesselation, which, however, belongs to the skin, and can be stripped from it. 

 When they have fairly passed the lips, they rapidly increase in size and hardness, and are 

 then developed into the tremendous crushing-mill which has already been described. 



The scales of fishes are divided by some writers into four classes. The first are 

 termed Placoid, or flattened scales, of which the common dog-fish affords a good example. 

 The next class is called Ganoid, or polished scales, such as those of the sturgeon. 



