52 BIVALVES. — OSTREA. 



The Oatrece may be divided into two classes: tlie fir»t 

 is that which comprises tlie innumerable varieties of es- 

 callops or scallop shells; the second (and much the least 

 numerous) is that which includes the species somewhat 

 resembling the common oyster. The former division is, 

 for the most part, composed of very elegant specimens; 

 their form is usually regular, and their surface is adorned 

 with elevated divergent ribs, in number from five to forty, 

 which proceed from the tip of the beaks, to the extremity 

 of the margins, and there terminate in a fine scalloped 

 or vandyked outhne, describing in their course the most 

 graceful expansion possible. It usually happens that the 

 Ostreae are inequivalve, that is, the degree of convexity 

 often differs in the two shells of the same animal, as in the 

 Ostrea ziczac, jacobtea, &c. &c. which invariably have 

 the upper valve flat, while the lower one remains of a 

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

 pallium, (ducal mantle), &c. &c. the shells are equivalve, 

 or both possessing the same degree of rotundity and gib- 

 bosity. 



It is remarkable that many of the Ostreae have the upper 

 valve adorned witli bright colors, and thelower 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 brownish or reddish cast. This species is also 

 in 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 Ostreae; 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 



