oD BIVALVES. OSTREA. 



convex form. However, in other species, as the Ostrca 

 Palhum (Ducal Mantle), &c. &c. the shells are equivalvc, 

 or both possessing the same degree of rotundity and gib- 

 bosity* 



It is remarkable that many of the Ostreae have the upper 

 Valve adorned with bright colors, and the lower paler: this 

 occurrence is particularly observable in the Ostrea Pleu- 

 ronectes, known by the name of the Compass or Floun- 

 der Pecten, which has one valve perfectly white, and the 

 other of a broAvnish or reddish cast. This species is also 

 HI possession of another peculiarity, which is, that it in- 

 variably gapes at both ends; whereas, in other species, 

 the gape is only at one end. 



The situation of the beaks often varies considerably 

 among the Ostrea) ; some, as the Maxima, Jacobaea, &c. 

 &c. having them placed in the centre; whereas, in the 

 Ostrea Lima, Glacialis,&c.they are situated obliquely or 

 on one side, which gives the outline of the shell an ap- 

 pearance of 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 others are une- 

 qual, havmg one much larger than the other; and some 

 are so small as hardly to be discernible. 



The ears of many of the Scallops are almost smooth on the 

 outside, whilst others have them rough, and even spined. 

 The Ostrea Pallium, and others, have one ear ciliate, and 

 are spined within. The exterior of the shells, as has been 

 before observed, is usually covered with elevated longitudi- 

 nal ribs and grooves, which are variously diversified with 

 beautiful colors and fine chequer- work. The ribs are 

 mostly covered with undulate and transverse striae, not 

 unfrequently assuming the appearance of elevated scales, as 



