14 



the Derwent Grilse all the vomerine teeth have disappeared 

 from the mesial line, a circumstance which never happens with 

 brown trout of similar weight and size, though in brown trout 

 of 10 or 15 lbs. weight from brackish water, these teeth are 

 sometimes absent. 



The caudal fin of a brown trout becomes square or truncate 

 when the fish attains a length of 10 or 12 inches. In the 

 Derwent Grilse, it is distinctly emarginate or forked. 



The number of scales on the line from the back of the 

 adipose fin, obliquely forward to the lateral line in the Derwent 

 Grilse is 12 ; in the brown trout it is always 15 or 16. 



As confirming the above, it must be remembered that the 

 Derwent Grilse was caught seven miles lower down the river 

 than any brown trout has yet been seen, and where the water 

 is too salt for that species to exist. 



It is, therefore, certain that we may discard the one species, 

 Salmo fario, from our consideration, and at once proceed to the 

 more difficult task of showing to which of the remaining two 

 the Derwent Grilse belongs. 



The formulae for the number of fin-rays of the two species 

 are given by Dr. Giinther as — 



Dorsal Anal Pectoral Ventral 



fin. fin. fin. fin. 



Salmon 14 11 14 9 



Salmon Trout 13 11 15 9 



It will thus be seen that the anal and ventral fins contain 

 the same number of rays in both species, but that the dorsal 

 fin contains one ray more, and the pectoral one ray less in the 

 salmon than in the salmon trout ; in the Derwent Grilse, the 

 dorsal and pectoral fins each contain 14 rays, thus agreeing 

 with the formula given for the true salmon. The weight of a 

 salmon trout in good condition, and of the same length as the 

 Derwent Grilse, would be more than 31bs., but the difference 

 between salmon and salmon trout in this respect is after all 

 only slight. 



In colour the Derwent Grilse agrees better with the majority 

 of specimens of Salmo salar than with the majority of specimens 

 of Salmo trutta, as it presents none of the purplish tinge so 

 generally found in the last mentioned species, nor has it the 

 greenish hue of the back common to most specimens of salmon 

 trout, but it must be borne in mind that specimens of Salmo 

 trutta do occur (especially amongst the females), agreeing 

 remarkably in colour with the true salmon. Specimens of 

 Salmo trutta of the size of the Derwent Grilse are usually more 

 densely covered with spots than is the case with the latter 

 fish. 



As to the constant characters, the whole form of the gill 



