Fig. 28. Head of female salmon.]
The most curious point about the male salmon is that during the
breeding-season, besides a slight change in colour, the lower jaw
elongates, and a cartilaginous projection turns upwards from the point,
which, when the jaws are closed, occupies a deep cavity between the
intermaxillary bones of the upper jaw. (7. Yarrell, History of
British Fishes, vol. ii. 1836, p. 10.) (Figs. 27 and 28.) In our
salmon this change of structure lasts only during the breeding-season;
but in the Salmo lycaodon of N.W. America the change, as Mr. J.K. Lord
(8. The Naturalist in Vancouver s Island, vol. i. 1866, p. 54.)
believes, is permanent, and best marked in the older males which have
previously ascended the rivers. In these old males the jaw becomes
developed into an immense hook-like projection, and the teeth grow into
regular fangs, often more than half an inch in length. With the
European salmon, according to Mr. Lloyd (9. Scandinavian Adventures,
vol. i. 1854, pp. 100, 104.), the temporary hook-like structure serves
to strengthen and protect the jaws, when one male charges another with
wonderful violence; but the greatly developed teeth of the male
American salmon may be compared with the tusks of many male mammals,
and they indicate an offensive rather than a protective purpose.
The salmon is not the only fish in which the teeth differ in the two
sexes; as this is the case with many rays. In the thornback (Raia
clavata) the adult male has sharp, pointed teeth, directed backwards,
whilst those of the female are broad and flat, and form a pavement; so
that these teeth differ in the two sexes of the same species more than
is usual in distinct genera of the same family. The teeth of the male
become sharp only when he is adult: whilst young they are broad and
flat like those of the female. As so frequently occurs with secondary
sexual characters, both sexes of some species of rays (for instance R.
batis), when adult, possess sharp pointed teeth; and here a character,
proper to and primarily gained by the male, appears to have been
transmitted to the offspring of both sexes. The teeth are likewise
pointed in both sexes of R. maculata, but only when quite adult; the
males acquiring them at an earlier age than the females. We shall
hereafter meet with analogous cases in certain birds, in which the male
acquires the plumage common to both sexes when adult, at a somewhat
earlier age than does the female. With other species of rays the males
even when old never possess sharp teeth, and consequently the adults of
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