242 My. 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 in 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 toru 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 mdustrious 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 hmits. 
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 
