GENERAL OUTLINES OF ICHTHYOLOGY. 31 



The form of the gill- covers is of great importance, to- 

 gether with the fins and teeth, in deciding the classification 

 of fish, and in some cases, as in the Salmonida, constitutes 

 one of the principal means of identifying different species 

 of the same genus. 



Fins. 



In considering the structure of fish, fins occupy a very 

 prominent place, not only as organs of motion, but as af- 

 fording, by their texture, position, and number, materials 

 for distinguishing orders, families, and genera. The mem- 

 branes of the fins are usually thin and more or less trans- 

 parent, and are supported by slender bony processes, or 

 props, called fin-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, soft or flexible rays. Upon this 

 difference in structure the two leading divisions of bony 

 fishes are founded. 



The numbers of fin-rays differ entirely in various species, 

 and, occasionally, in different individuals of the same 

 species. 



The names of the fins are derived from the parts of the 

 body to which they are most commonly attached, and a 

 reference to the annexed diagram will assist in impressing 

 these upon the memory of the reader. 



They are : 



