PISCES. 97 



GENUS DIODON, Lin. 



Jaws undivided, and presenting only one piece above and 

 another below, behind the trenchant edge of which is a round 

 portion transversely furrowed, and forming a powerful in- 

 strument of mastication ; skin armed on all sides with thick 

 pointed spines, so that when inflated they resemble the burr 

 of a chestnut tree. They can also swell themselves out by 

 filling their stomach with air, or rather a sort of fine and ex- 

 tensible crop, which occupies the whole length of the ab- 

 domen. 



FAMILY II. SCLERODERMI. 



Snout conical or pyramidal, prolonged from the eyes, and 

 terminated by a small mouth armed with a few distinct teeth 

 in each jaw; skin generally rough or covered with hard 

 scales. One remarkable genus. 



GENUS OSTRACION, Lin. 



In place of scales bony and regular compartments, soldered 

 as- in a cuirass, permitting no free movement except to the 

 tail and fins, which pass through the holes of this corselet. 



Series H. Chondropterygii. Cartilaginous Fishes. 



Skeleton consisting of a mere homogeneous and semitrans- 

 parent cartilage, which is invested only at the surface, in the 

 genera Raia and Squalus, with a bed of small, opaque, circular 

 grains, arranged one against the other and not in threads or 

 filaments. 



In the Lampreys the skeleton has not even this envelope; 

 and, in the Ammoccetes, it remains absolutely membranous. 



The most apparent characteristic of this division of the 

 class of Fishes is the absence of maxillary and intermaxillary 

 bones ; or rather in their only having vestiges of them under 

 the skin, while their functions are fulfilled by bones analo- 

 gous to palatines, and sometimes even by a vomer. Three 

 orders. 



N 



