THE SALMON AND SEWIN. 197 
sufficiently exact criterion for determining their‘relative 
ages within certain limits. 
Smolts of the Salmon and Sewin have the caudal 
fin much forked; but a progressive alteration in the 
shape of this organ is effected by the more rapid elon- 
gation of its central rays as the fish advance in growth, 
till, on the acquirement of its perfect development, 
the posterior margin becomes nearly straight in the 
Salmon, and actually somewhat curved outwards in 
large males of the Sewin, thus supplying the means 
of forming a comparative estimate of the ages of both 
species. 
Tn accordance with what is here stated, I find that 
specimens weighing from half a pound to a pound and 
a half have the caudal fin more or less forked and the 
vomer wel] supplied with teeth except at its posterior 
part, from which some are lost invariably. Specimens 
weighing from two to five pounds have the posterior 
margin of the caudal fin either moderately forked or 
nearly straight, according to their size, and usually 
have from three to seven or eight teeth on the anterior 
part of the vomer, the number, after making a suitable 
allowance for differences in condition, being almost 
always inversely as the weight; and individuals of 
large dimensions constantly have the posterior margin 
of the caudal fin nearly straight or slightly curved 
outwards in males of the Sewin, and retain one or 
two teeth only at the anterior extremity of the vomer, 
or are even without any. 
