SALMONIDiE. 



635 



over, one and the same individual may pass a part of its 

 life in a lake, and enter a river at certain periods, thus 

 changing the form of its fins almost periodically. 



Finally, to complete our enumeration of these variable 

 characters, we must mention that in old males, during and 

 after the spawning-season, the skin on the back becomes 

 thickened and spongy, so that the scales are quite invisible, 

 being imbedded ia the skin. 



After this cursory review of variable characters we pass 

 on to those which are more constant, not subject to ready 

 modification by external circumstances ; and which, therefore, 

 ought to be noticed in every description of a species of Salmo. 



1. The form of the prceoperculum of the adult fish. — The 

 prseoperculum is composed of a vertical (posterior) and hori- 

 zontal (lower) part (limb), both meeting at a more or less 

 rounded angle. The development of the lower limb is a very 

 constant character ; in some species (as in the Salmon) it is 

 long, in others (*S^. ferox, S. Irachypoma) exceedingly short. 

 The adjoining woodcuts will readily show this difference. 



A B 



Fig. 287. — Prffioperculum of A. Salmo salar ; B. Salmo brachypoma. 



In young specimens of all Salmonoids the preeoperculum 

 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. 



