620 FOURTH REPORT — 1834. 



large black and ocellated spots upon all the superior parts of 

 its body ; but from the third year they diminish, and ere long 

 they entirely disappear. 



The Salmo Umbla, so long as it is young, is of a greenish 

 yellow colour, with the abdomen white ; and at a later period of 

 life these tints assume a deeper hue of a more lively green, and 

 finally pass into a blackish green. The abdomen soon becomes 

 silvery white, afterwards yellow and orange coloured, and then 

 of a golden lustre. Its flanks are very soon adorned with ocel- 

 lated yellow spots, more or less distinct ; but ere long there are 

 no spots at all. In the Salmo Fario the spots vary even more. 

 In the young they are found yellow, green, brown, and even 

 black and violet, also black and red ; but at length they all 

 entirely disappear. 



The author has also noticed that the seasons have an influ- 

 ence on the colours of the different kinds of Salvia. 



It is during the autumn, and at the time of the greatest cold, 

 that is to say in October, November, December, and January, 

 that their tints are most brilliant, and the colours become more 

 vivid by the accumulation of a great quantity of coloured pig- 

 ments. We might almost say that these fishes bedeck them- 

 selves in a nuptial garb as birds do. The colour of their flesh 

 varies according to the nature of their aliment. This family of 

 fishes feeds, as we have said above, especially upon the larvae 

 of aquatic insects and of small Crustacea. It is in the waters 

 which contain the most of these last that the most beautiful 

 salmon-trout are found. Direct experiments which were made 

 in lakes have proved, to the author's satisfaction, that the in- 

 tensity of the colour of the flesh arises from the greater or smaller 

 quantity of Gammarina; which they have devoui'ed. 



As to the structure of the head, it offers, in the opercular 

 bones, in the surface of the cranium, and in its proportions re- 

 lative to the whole body, very excellent characters : but those, 

 on the other hand, which are taken from the proportional 

 length and size of the jaw-bones are of no value at all ; the 

 lower jaw is longer or shorter than the upper according as 

 the fish opens or shuts its mouth ; and this consideration in- 

 troduced into the characteristics of the family has very con- 

 siderably contributed to multiply the institution of species. 

 The hook which forms the jaw of the Salmo Salar is not even 

 a peculiar characteristic of this species, since the full-grown 

 males of all the species of the genus present a crooked prolon- 

 gation of their lower jaw to a greater or less extent. 



Possessed of these facts, which had been collected with the 

 most minute and jealous precautions, M. Agassiz tried to deter- 



