OUTLINES OF ICHTHYOLOGY. 169 



the sub-operculuni, or the under gill-cover, c ; and the 

 iW^y-tf/^rf?^/?/;;/, or intermediate gill-cover, d. The 

 three last of these are articulated, or jointed on, to 

 the temporal bone, and play upon th^ prc-opennlitiii 

 (a fixed bone), a ; and by the opening and shutting 

 of these gill-covers respiration is carried on. 



The form of the gill-covers is of great impor- 

 tance, together with the fins and teeth, in deciding 

 the classification of fish. 



The braiichiostcgous rays, or gill-rays, marked 

 E, form a bony fan-like appendage, or fringe, to the 

 lower margin of the gill-covers, to which they are 

 attached. 



The Fins occupy a very prominent place in fish 

 structure, not only as organs of motion but as 

 affording by their nature, position, and number, 

 materials for distinguishing orders, families, and 

 genera. 



The membranes of the fins are usually thin and 

 more or less transparent, and are supported by 

 slender bony processes, or props, called Jin-rays, 

 some of which are composed of single pieces, 

 pointed at the end, designated spinous rays, from 

 their resemblance to spines and thorns : and others 

 of a number of separate pieces united by joints, 

 and called, from their pliant nature, sofi ox flexible 

 rays. 



Upon this difference In structure the two leading 

 divisions of bony fishes are founded. 



