10 SALMONIDiE. 



to occur. The cause of these irreg-ular ascents previous to the 

 autumnal ascent is unknown. A part, at least, of the hybrid fishes 

 retain the migratory instinct : but it is not known whether sterile 

 inchviduals accompany the others on their migrations. 



5. It is said that the migratory species invariably return to the 

 liver in which they were bred. Experiments have shown that this is 

 normally the ease ; but a small proportion appear to stray so far away 

 from their native place as to be unable to find theii' way back. Thus, 

 several specimens of Sea-Trout, which have been caught Avith Sewin 

 and Salmon in South Wales, difler so remarkably from any other 

 species known to ourselves, that we cannot help thinking that they 

 are stray individuals of a kind of Trout (see also p. 86) and from a 

 locality unknown to us at present. Further, almost every year Salmon 

 and Sea-Trout in the Grilse state make their appearance at the mouth 

 of the Thames (where the migratory Salmonoids have become extinct 

 for many years), ready to ascend and to restock this river as soon as its 

 poisoned water shall be sufficiently purified to allow them a passage. 



6. There has been much dispute about the time required for 

 the growth of Salmonoids. The numerous and apparently contra- 

 dictory observations tend to show that there is a great amount of 

 variation even among individuals of the same origin living under the 

 same circumstances, some of them growing much more quickly than 

 others, and being ready to descend to the sea twelve months before 

 their brethren. The cause of this irregularity is not explained. On 

 the other hand, when we consider the fibrous condition of the Sal- 

 monoid skeleton, which is much less solid, and more wanting in 

 calcareous substance, than that of the majority of Teleosteous fishes, 

 we shall be quite prepared to adopt the truth of the observation 

 that the young Salmonoids return to the fresh water after a few 

 months' sojourn in the sea, and after having feasted on nourishing 

 Crustaceans, Sand-eels, &c., with their former weight in ounces 

 increased to pounds. 



After these preliminary remarks we proceed to the enumeration of 

 the species. We shall see that it is impossible to give the distinctive 

 characters of a great proportion of the species, which are known from 

 antiquated or popular descriptions only. In these cases the locality 

 will be the best guide for recognizing the species. However, we can 

 nearly always decide whether the fish is a Salmo proper, or whether 

 it belongs to the Charr-group. Therefore, after having- divided, 

 with Nilsson, these fishes into those two groups, we arrange the 

 species of each geographically. 



First Subgeneric Group, Salmones. 



Truttfe, Xikson, Proih: Ichth. Scand. pp. 2 and 7. 

 Salar et Fario, Cuv, 8f Val. 



Teeth not only on the head of the vomer, but also along its body ; 

 in a few species the posterior teeth arc lost with age, and only a 



