58 BIVALVES——_OSTREA. 
of the upper valve are brighter than those of the lower, 
as may be particularly observed in the O, pleuronectes, 
which has one valve perfectly white, and the other of a 
brownish or reddish cast. 
The beaks of the Scallops often vary considerably 
in position; some, as the O, maxima, O. jacobea, &c. 
have them placed in the centre; while, m the O. lima, 
O. glacialis, &c. they are situated obliquely or on one 
side, which gives the outline of the shell an appearance 
of slight distortion, occasioned by one margin bemg 
straight and flattened, and the other round and inflated. 
There is also considerable variation in the size and 
form of the ears, which in some species are nearly of 
the same dimensions, but in others are unequal; and 
some are so small as hardly to be discernible. 
The ribs are variously diversified with beautiful colors, 
and delicate chequer-work; they are usually covered 
with undulated and transverse striz, not unfrequently 
assuming the appearance of elevated scales, as in the 
O. imbricata and O. dubia. In others the striz are 
crenated, as in the O. radula; and some, as the O. no- 
dosa, &c. have large knobs or tubercles raised upon the 
ribs. The margins of the interior of the Scallops are 
mostly crenated, and are often beautifully colored. 
The hinge is universally without teeth, and is fur- 
nished with an ovate hollow; in the vicinity of which 
are placed lateral transverse grooves, which run ina 
parallel direction in either valve, but do not lock mto 
each other, as in the genus Arca. 
The Scallops have the faculty of leaping out of the 
water, and are enabled to effect a very rapid motion by 
opening and closing their valves. 
