SALMONID^. 631 



Mallotiis, indistinguishable from the living M. villosus, occurs 

 abundantly in nodules of clay of unknown geological age in 

 Greenland. Other genera, as Osmeroides, Acrognatlmis, and 

 Aulolepis, from the chalk of Lewes, belong to the same fauna 

 as species of Beryx, and were probably deep-sea Salmonoids. 



Salmo. — Body covered with small scales. Cleft of the 

 mouth wide, the maxillary extending to below or beyond the 

 eye. Dentition well developed ; conical teeth in the jaw bones, 

 on the vomer and palatines, and on the tongue, none on the 

 pterygoid bones. Anal short, with less than fourteen rays. 

 Pyloric appendages numerous ; ova large. Young specimens 

 with dark cross-bands (Parr-marks). 



We know of no other group of fishes which offers so many 

 difficulties to the ichthyologist with regard to the distinction 

 of the species as well as to certain points in their life-history, 

 as this genus, although this may be partly due to the unusual 

 attention which has been given to their study, and wdiich has 

 revealed an almost greater amount of unexplained facts than 

 of satisfactory solutions of the questions raised. The almost 

 infinite variations of these fishes are dependent on age, sex 

 and sexual development, food, and the properties of the 

 water. Some of the species interbreed, and the hybrids mix 

 again with one of the parent species, thus producing an off- 

 spring more or less similar to the pure breed. The colora- 

 tion is, first of all, subject to variation ; and consequently 

 this character but rarely assists in distinguishing a species, 

 there being not one which would show in all stages of de- 

 velopment the same kind of coloration. The young of all 

 the species are tarred ; and this is so constantly the case that 

 it may be used as a generic or even as a family character, not 

 being peculiar to Sahno alone, but also to Thymallus and 

 probably to Corcgonus. The number of bars is not quite 

 constant, but the migratory Trout have two (and even three) 

 more than the Biver- Trout. In some waters Eiver- trout 



