GKILSE 45 



that one or two, such as Thymallus (the graylings), 

 are purely fresh water, and that the genera Salmo 

 and Coregonus include species which are both marine 

 and fresh water in habit. It may fairly be noted, 

 however, that the species of the two last named genera, 

 usually considered as inhabitants of fresh w^ater alone, 

 can somewhat readily be accustomed to the sea — that 

 indeed any salmonid may apparently be accustomed 

 to salt water — whereas a pure fresh water fish, such 

 as a carp, at once succumbs to the effects of salt 

 water. Also we have Osmerus (the smelt or spar- 

 ling), which is classed along with four other genera, 

 three of which are marine, coming into our estuaries 

 to spawn. In the allied group of fishes, including 

 the herrings, we also see the herring itself, the sprat, 

 and the shads coming freely into estuaries, and the 

 shads spawning in fresh water. 



If, further, we consider the salmonids, let us say 

 the British salmonids, which are commonly found 

 in fresh water, and note their characteristics in 

 relation to the common developmental characteristics 

 of the genus Salmo, we may observe that certain 

 features common to salmon parr, the so-called parr 

 marks and the complete dentition, very usually 

 survive throughout the life of the fresh water form. 

 If salmon smolts are retained in fresh water beyond 

 their natural time of descent to the sea, they assume 

 again the trout-like appearance of the juvenile. 

 We have already seen in the previous chapter that 

 the Howietoun specimens which spawned, and which 

 were figured by Day in the Transactions of the 

 Linnsean Society, retained the parr marks. The land- 



