BIVALVE MOLLUSKS. 223 
duction, with a milky fluid containing multitudes 
of small granules of a whitish colour. These are 
the eggs; and in many of the family they are not 
at the time of their exclusion abandoned at once, 
but are deposited between the membranes of the 
gill-leaves, where they undergo a kind of incuba- 
tion. In some, the shell is developed in the egg 
before it quits this receptacle. This fostering of 
the eggs seems to be analogous to the gestation of 
the eggs in the Crustacea and the Pipe-fishes.* 
I have hitherto spoken of but a single adductor 
muscle, but in a large number of species there is 
a second, placed near the front part of the animal. 
This variation naturalists have used to divide the 
Class into two Orders, denominated Dimyaria and 
Monomyaria, or respectively. Double and Single- 
muscled Bivalves. ‘l'hese characters can be de- 
termined at a glance by looking at a single valve 
of any shell; for the place of attachment of the 
adductor muscle is marked by a distinct sunken 
impression in the interior of each valve. From 
this circumstance also, these impressions, by the 
differences in their form, position, and dimensions, 
afford excellent characters for the discrimination 
of genera. 
The accompanying figure, representing the 
interior of one of the valves of a common shell, 
will serve to illustrate the appearance of the mus- 
cular impressions, as well as of some other parts 
that are commonly mentioned in technical deserip- 
tion. ‘The oval mark on the left-hand side of the 
figure is the front muscular impression ; the pear- 
shaped mark on the opposite side is the hinder 
one; the bending line which connects them is 
* Penny Cyclop. vil. 432. 
