THE SHELL-FISH OF THE COAST. 45 
OYSTERS, SCALLOPS, MUSCLES, AND ARKS. 
The oyster is so familiar to everybody that it 
scarcely needs description. Still, there are a num- 
ber of points connected with its structure and _his- 
tory which may not be generally known, and may 
consequently be touched upon with advantage. In 
the first place, let it be said that there are two gen- 
erally recognized species or varieties on our coast, 
—one known as the Virginia oyster, of an elongated 
form, and the other, deeply scalloped, the Northern 
oyster (Ostrea borealis). But the shell of the oyster 
varies so greatly, depending for its form so much 
upon the shape of the object upon which it immov- 
ably attaches itself in later life, that it becomes a 
matter of great difficulty to determine the proper 
limits of specific variation; and, indeed, as far as 
the two forms above noted are concerned, it is very 
doubtful if they do not in reality belong to a single 
species. In both, as in nearly all oysters, the left 
valve is the larger of the two, and it is upon this 
that the animal rests. The two are brought to- 
gether by means of a single powerful muscle (ad- 
ductor), whose attachment to the shell leaves the 
dark sub-central impression which is frequently 
ealled the ‘ heart.’ 
Oysters are marine in habit, but they seem able 
to endure a certain amount of exposure to fresh 
water, as in the mouths of estuaries and bays, 
which constitute their chief abiding-place. The 
‘banks’ or ‘reefs’ rise to within a few feet of the 
surface, and in many regions are even exposed 
