BIVALVES. OSTREA. 53 



niiuate in a fine scalloped or Vandyked outline, describ- 

 ing in their course the most graceful expansion possible. 

 The Ostreae are usually inequivalve, that is, the degree 

 of convexity often differs in the two valves of the same 

 specimen, as in the O. ziczac, O.jacobsea, &c. which in- 

 variably have the upper valve flat, and the lower convex. 

 However, in other species, as the O. pallium, (ducal man- 

 tle), &c. the shells are equivalve, i. e. both possess the 

 same degree of rotundity and gibbosity. 



It is remarkable, that in many of the Ostreoc, the co- 

 lors of the upper valve are brighter than those of the. 

 lower: this is particularly observable in the O. pleuro- 

 nectes, known by the name of the compass or flounder 

 Pecten, which has one valve perfectly white, and the 

 other of a brownish or reddish cast. This species has 

 also another peculiarity, viz. that it invariably gapes at 

 both ends; whereas, in other species, the gape is only 

 at one end. 



The situation of the beaks often varies considerably; in 

 some, as the O. maxima, O. jacobaea, &c. they are placed 

 in the centre; whereas, in the O. lima, O. glacialis, &c. 

 they are situated obliquely or on one side, which gives 

 the outline of the shell an appearanceof slight distortion, 

 occasioned by one margin being straight and flattened, 

 while the other remains 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 many, one is much larger than 

 the other; and in a few, they are hardly discernible. 



The ears of many of the scallops are almost smooth on 

 the outside, but some are rough, and even .spined. The 

 O. pallium and others have one ear ciliate, and are spin- 

 ed within. 



