48 



SALMONID^. 



with which alone we have to do, consist of four principal parts, 

 and their use is to close the aperture behind the gills, which in 

 all these three divisions is so formed, and so freely or loosely 



Jma^ 



suspended, that the water bathes in its passage every part of 

 their surface. 



These parts are, the pre-operculum, or fore-gill-cover, No. 1 ; 

 the operculum, or gill-cover proper. No. 2 ; the sub-operculum, 

 or under-gill-cover, No. 3 ; and the inter-operculmn, or inter- 

 mediate gill-cover, No. 4. The branchiostegous rays, as they are 

 termed, are indicated by No. 5 ; and the fixed plates, forming 

 the posterior immoveable margin of the gill-covers, by No. 6. 

 No. 7 indicates the pectoral fin. 



How widely these parts differ in form, in diff'erent species of 

 the Salmon tribe, will become at once apparent by a comparison 

 between the gill-covers in the figure above, and those of the 

 true Salmon [Salmo Salar), and the Bull Trout {Salmo Eriox), 

 Nos. 2 and 3, in the following cut, which, with these, presents 

 a view of the interior of the mouth and the dental system of the 

 Common Trout {Salmo Fttrio), of Great Britain. 



