350 CARTILAGINOUS FISHES. 



or granular skin (not bony) ; they have eight teeth, 

 generally trenchant, arranged in a single row in each 

 jaw, and two dorsal fins. They are found in great 

 numbers in the Torrid zone. Others, called 



Trunk Fishes (Ostracion),* have, instead of scales, 

 an inflexible coat of mail made up of bony plates, 

 which covers the head and body, so that they can 

 only move their tails, their fins, and their mouth, all 

 of which protrude through apertures in their remark- 

 able armour. Each jaw is armed with ten or twelve 

 conical teeth. They are common on the coast of 

 America. 



DIVISION OF CARTILAGINOUS FISHES. 



CHONDROPTERYGII.t 



The Chondropterygii differ from all the fishes we 

 have as yet spoken of in the following particulars. 

 Their skeleton is cartilaginous, and always more 

 simple in its conformation than that of osseous fishes. 

 The skull is composed of a single piece, but shaped 

 in other respects very much like that of an ordinary 

 fish. The maxillary and intermaxillary bones do 

 not exist, or are found only in a rudimentary state, 

 concealed beneath the skin. The lower jaw is con- 

 stituted of one piece on each side, and the opercular 

 apparatus is in general entirely wanting. 



Sometimes the gills are free on their external edge, 

 as in the osseous fishes ; sometimes, on the contrary, 

 they are attached by both edges, and from this cir- 

 cumstance the Chondropterygii are divided into two 

 groups. 



1. Those with free branchiae (Sturgeons). 



2. Those with fixed branchiae (Sharks, Kays, &c.). 



The Sturgeons (Acipenser) have the general form of osseous 

 fishes ; their body is more or less covered with plates of bone 



* offrpaKov,, ostracon, a shell. 



f X<^8pos, chondros, cartilage; irTeptyiov, pterygion, a fin. 



