GENERAL FEATURES 57 



the reader that the point is in fact material as a mark of distinction, at 

 any rate between salmon and trout. 



Such bearing as the gill-covers have upon the matter in hand may 

 best be explained by reference to the diagrams of the heads of salmon 

 and sea-trout (Figs. 13 and 14) here given. 



In the salmon, as Yarrell noted, the posterior free edge of the 

 gill-cover formed by the operculum, subopercultwi and inler operculum 

 is almost invariably of so true a curve as to form part of a circle. In 

 the sea-trout, and, one may say, in the trout also, the free edge, where 

 the junction of the operculum with the suhoperculum occurs, projects 

 backwards as a rule beyond the line of the curve of a true circle. One 

 cannot say that the distinction is absolute or constant, for the curve in 

 each species may approximate sometimes so closely as to be indistin- 

 guishable; but in most cases the true curve of the gill-cover of the 

 salmon will be easily marked off from the more projecting curve of the 

 gill-cover of the sea-trout. 



Something also falls to be stated regarding the teeth in this connec- 

 tion because, although the young salmon, sea-trout and trout, begin life 

 with equally well-armed jaws, most of the teeth of the salmon disappear 

 as the fish attains maturity. As a distinction in mature fish of the two 

 species we are considering the teeth therefore are of some importance. 

 Yarrell here again puts the matter very clearly. " I have observed," 

 he says, " that some specimens of the migratory or Sea Trout carry 

 their vomerine teeth longer than the Salmon ; and the Trout which do 

 not migrate appear to carry their vomerine teeth longer than those which 

 do migrate." He remarks that the salmon loses a portion of the 

 vomerine teeth during the first visit to salt water, and states further : — 

 " The teeth on the vomer of the Salmon, when the fish is old, seldom 

 exceed two or three in number, sometimes only one, and that placed 

 on the most anterior part." It may thus be inferred that any large fish 

 whi( h discloses a fairly complete set of teeth on tongue, jaw and vomer 

 is not a salmon. 



