BOOK II. 



OF CARTILAGINOUS FISHES. 



CHAP. I. 



OF CARTILAGINOUS FISHES IN GENERAL. 



We have seen that fishes of the cetaceous kind bear a strong re- 

 semblance to quadrupeds in their conformation ; those of the 

 cartilaginous kinds are one remove separated from them ; they 

 form the shade that completes the imperceptible gradations of 

 nature. 



The first great distinction they exhibit is, in having cartilages 

 or gristles instead of bones. The cetaceous tribes have their 

 bones entirely resembling those of quadrupeds, thick, white, and 

 filled with marrow ; those of the spinous kind, on the contrary, 

 have small slender bones, with points resembling thorns, and 

 generally solid throughout. Fishes of the cartilaginous kinds 

 have their bones always soft and yielding ; and age, that hardens 

 the bones of other animals, rather contributes still more to soften 

 theirs. The size of all fishes increases with age ; but from the 

 pliancy of the bones in this tribe, they seem to have no bounds 

 placed to their dimensions ; and it is supposed that they grow 

 larger every day till they die. 



They have other differences, more obviously discernible. We 

 have observed, that the cetaceous tribes had lungs like quadru- 

 peds, a heart with its partition in the same manner, and an ap- 

 paratus for hearing ; on the other hand, we mentioned that the 

 spinous kinds had no organs of hearing, no lungs to breathe 

 through, and no partition in the heart ; but that their cold red 

 blood was circulated by the means of the impulse made upon 

 their gills by the water. Cartilaginous lihlics unite both these 



