i68 OUTLINES OF ICHTHYOLOGY. 



Perch both these bones and the pterygoid carry 

 teeth. The lower jaw, except in the cartilaginous 

 fishes, has generally two bones in each side : a 

 further reference to these bones will be found 

 under the Division " Teeth." 



The relative positions of the bones composing 

 the gill-covers, or opercula, will perhaps be more 

 easily distinguished by a reference to the accom- 

 panying diagram of the head of a Trout, in which 



A represents ti\t pre-operculum, or fore-gill cover. 

 B, the operculum, or gill-cover proper. 

 c, the sub-operculum^ or under gill-cover. 



D, the inter-operciihim, or intermediate gill-cover. 



E, the branchiostegous rays, or gill rays. 1 



It will thus be seen that the gill-covers consist 

 of four pieces each : \hzpre-opercidum, or foregill- 

 cover, A ; the operculum, or gill-cover proper, B ; 



1 From the Greek branchia, gills, and stegem, to cover. 



