INDIAN 8ILLAGO. Stftojpo ddmina. 



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 comparative 

 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 alto- 

 gether too abstruse a subject to be admitted into these pages, in spite of its exceeding 

 interest. 



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. 



