Fig. 290.—Vomerine teeth of Salmo fario, lower view.
Fig. 291.—Vomerine teeth of a Charr, side view.
5. The form of the caudal fin in specimens of a given size, age, and sexual development.
6. A great development of the pectoral fins, when constant in individuals from the same locality.
7. The size of the scales, as indicated by the number of transverse rows above the lateral line: one of the most constant characters.
8. The number of vertebræ.—Considering the great number of vertebræ in Salmonoids the constancy of this character is truly surprising. An excess or a diminution of the normal number by two, is of rare occurrence, and generally to be explained by the fact that one vertebra has been abnormally divided into two, two such vertebræ being considerably smaller than the others; or, on the other hand, that two have merged into one centrum, which is then unusually large, and provided with two neural spines. We have seen one case only, in which three vertebræ were united. The number of vertebræ can be easily ascertained in specimens destined for preservation in spirits, by an incision made along one side of the fish, a little above the lateral line.
9. The number of pyloric appendages.—There can be no doubt that this character may materially assist in fixing a species. We shall see that in some species it varies from 30 to 50; but in others, as in the Salmon and Charr, it has been found very constant (see Fig. [56], p. 131). If unexpected variations occur, their cause may be found in a partial confluence of the cæca, as we have observed that specimens of S. levenensis (a species normally with from 70 to 90 cæca), had those appendages of unusual width when the normal number was diminished.
We have mentioned above that many points in the life-history of the Salmonoids still remain very obscure:—