BIVALVES. — OSTREA. 61 



The Ostreae are divided into several classes : the first and 

 second comprise the innumerable varieties of escallops or 

 scallop shells, and are distinguished from each other by the 

 proportion of their ears ; the third and remaining divisions 

 include the species somewhat resembling the common 

 oyster. 



The first 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 ter- 

 minate in a fine scalloped or Vandyked outline, describ- 

 ing in their course the most graceful expansion possible. 



The greater number of the species of this genus are in- 

 equivalve, that is, the degree of convexity differs in the two 

 valves of the same specimen, as in the O. ziczac, O. jaco- 

 bsea, &c. which invariably have the upper valve flat, and the 

 lower convex. However, in other species, as the O. pallium, 

 (ducal mantle), &c. the shells are equi valve, i. e. both pos- 

 sess the same degree of rotundity and gibbosity. 



It is remarkable, that in many of the Ostreee the colours 

 of the upper valve are brighter than those of the lower : 

 this is particularly observable in the O. pleuronectes, 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 appearance of slight distortion, oc- 

 casioned by one margin being straight and flattened, while 

 the other remains round and inflated. 



