242 Mr. J. Blackwall on the Salmon and Bull-trout. 



stream, more especially to kelts, which are comparatively of little 

 value ; and that this is not merely a supposititious case, or an 

 imaginary cause of delusion, I can confidently affirm from personal 

 experience. Perforations, and the total or partial excision of any 

 of the fins, may be objected to on account of the modifications 

 which such marks undergo with the growth of the fish, and also 

 on account of the mutilations to which those members are liable 

 from incidental circumstances. 



Having thus succinctly directed attention to a few of the ob- 

 jections which may be urged against the manner in which 

 attempts to ascertain the rate of growth in fish by employing 

 artificial marks are generally conducted, I shall revert to the 

 method pursued m my own researches, already referred to at 

 the commencement of this article ; namely careful and frequently 

 repeated observations on the gradual loss of the teeth from the 

 vomer, on the order in which they are shed, and on the changes 

 known to take place in the figure of the caudal fin. 



The usual number of teeth on the tongue of the salmon-smolt 

 and bull-trout- smolt of six or seven inches in length, when none 

 has been lost, is ten, arranged in a row of five on each side ; 

 occasionally I have counted as many as twelve in both species, 

 but ten appears to be the normal number. These teeth are not 

 shed, but most of them are torn away by violence in an irregular 

 manner as the fish advance in growth, so that a want of symmetry 

 in the two rows is conspicuous in much the greater number of 

 individuals. I may remark that such is the case also in every 

 particular with the teeth on the tongue of the common trout, 

 Salmo fario. 



The teeth on the vomer of the salmon-smolt and bull-trout- 

 smolt commonly exceed twenty (in numerous instances I have 

 noticed twenty-four), a fact which the minute inspection of the 

 heads of both species, after having been placed in nests of the 

 great wood-ant, Formica rufa, and subjected to the anatomical 

 process so admirably effected by that industrious insect, fully 

 confirms. Unlike the teeth on the tongue, those on the vomer 

 are shed gradually, commencing at the posterior part and dis- 

 appearing in nearly regular succession as the fish increase in size ; 

 consequently, the loss of teeth from the vomer, taken in con- 

 junction with the form of the tail and the growth of these species, 

 affords to experienced observers a sufficiently exact criterion for 

 determining their relative ages within certain limits. 



Smolts of the salmon and bull-trout have the caudal fin much 

 forked ; but a progressive alteration in the shape of this organ is 

 effected by the more rapid elongation of its central rays as the 

 fish advance in growth, till, on the acquirement of its perfect 

 development, the posterior margin becomes straight in the 



