In young specimens of all Salmonoids the præoperculum has a very short lower limb; but whilst in some species it lengthens with age, its development in a horizontal direction is arrested in others.
2. The width and strength of the maxillary of the adult fish.—To show this character in two distinct species, we have given woodcuts of the maxillaries of females (12 inches long) of S. fario and S. levenensis of the same size.
Fig. 288.—Maxillary of A. Salmo fario; B. Salmo levenensis.
In young specimens of all Salmonoids the maxillary is comparatively shorter and broader, somewhat resembling that of Coregonus; yet this bone offers a valuable character for the determination of the young of some species; for instance, in a young S. cambricus it extends scarcely to below the centre of the eye, whilst in S. fario of the same size it reaches to, or even beyond, this point.
Fig. 289.—Vomerine teeth of Salmo salar (Salmon). A. Side view. B. Lower view.
3. The size of the teeth, those of the intermaxillaries excepted.
4. The arrangement, and the permanence or deciduousness of the vomerine teeth.—In some species the vomer is normally armed with a double or single series throughout life, although, of course, some of the teeth are frequently accidentally lost; in others, these teeth disappear gradually with age, the hinder ones first, so that finally the anterior only remain. In order to ascertain the arrangement of the teeth, it is necessary to remove the gengiva. Frequently the teeth stand in a distinctly double or single series, or they are placed alternately; but frequent irregularities occur which render the character vague, or even unsafe, so that some zoologists have rejected it entirely as unreliable. However, when a greater number of individuals really belonging to the same species are examined, a pretty safe conclusion may be arrived at as regards the arrangement of the teeth.
