156 SALMON AND TROUT. 
below. In both species the shape of the gill cover differs some- 
what from that of the bull trout, and also from that of the 
yellow trout (arto), the shape of the gill cover in which last- 
named species is shown in the engraving on page 155. 
The difference in the position &c. of the teeth, again furnishes 
a ready test for distinguishing the sea trout from the true salmon 
and from the bull trout. In the sea trout the teeth on the vomer, 
or central bone in the roof of the mouth (marked a in engraving) 
are more numerous than in either of the other species, and 
often remain, as shown, extending a considerable distance along 
a, teeth on vomer, or central bone of the roof of the mouth ; BB, teeth on right 
and left palatine bones ; c, row of hooked teeth on each side of the tongue ; 
DD, teeth on superior maxillary, or maxillary bones ; EE, teeth on lower 
jaw, or inferior maxillary bones. 
the bone, whilst in the true salmon and in the bull trout they 
are almost all lost upon the first migration to the sea, and only 
two or three left on the most forward end of the bone. Even 
these teeth, in very old fish, are frequently reduced to a single 
representative, or entirely disappear. 
In the sea trout also, however, the teeth on the vomer 
diminish in numbers as the fish gets older, and will often be 
found in a cluster only at the end of the bone; but they are 
always retained in greater numbers than in the true salmon and 
bull trout. The teeth generally of the sea trout are also finer 
