GRILSE 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 speciesof the two last named genera, 
usually considered as inhabitants of fresh water 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 
Linnzan Society, retained the parr marks. The land- 
