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. jacobaea, &c. 

 have them placed in the centre ; while, in the O. lima, 

 O. glacialis, &c. they are situated obliquely or on one 

 side, which g-ives the outline of the shell an appearance 

 of slight distortion, occasioned by one margin being 

 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 striae, not unfrequently 

 assuming the appearance of elevated scales, as in the 

 O. imbricata and O. dubia. In others the striee 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 in a 

 parallel direction in either valve, but do not lock into 

 each other, as in the genus Area. 



The Scallops have the faculty of leaping out of the 

 water, and are enabled to eflfect a very rapid motion by 

 opening and closing their valves. 



