13 
TAB. Il. 
AVICULA media. 
Ba Aika aR EE A 
Gen. Cuar. A free Shell, a little gaping near the 
beaks, fixing itself by a byssus, having valves 
of unequal size, the hinge linear, without a 
tooth, extended over the beaks into two wings. 
The Cartilage of the hinge oblong, broadest 
near the middle and marginal. Muscular 
impressions two, contiguous. 
Spec. Cuar. Ovate, depressed; wings large, un- 
equal, one wing acute, valves nearly equal; 
length of the Shell and hinge nearly equal ; 
surface smooth. 
C0 
Ovr Shell agrees sufficiently with the recent species of this 
genus to be ranked with them, and more correctly so than 
with those of any other genus. Its valves, however, appear 
to be nearly equal, and the hinge in the larger specimens 
shows some signs of a depression and tooth near the short 
wing, and a little elongated swelling on the opposite side, 
like Mytilis Hirundo Linn. We think also that there is 
some vestige of a constriction or fold around the short 
wing; that is most apparent in the large pearl muscle, 
Mytilus margaritiferus, Linn. Indeed it seems altogether 
very nearly allied to both of these, but is nevertheless a 
distinct species, the wings being shorter than in M. 
Hirundo, and longer than in M. margaritiferus. I there- 
fore consider it as a species which comes between the two, 
and name it accordingly. 
