BIVALVES. OSTREA. 35 



OSTREA.— Oyster anO Scallop. 



Animal— a Tetliys: Shell bivalve, generalh/ with xinequal 

 valves and slightly eared; hinge without teeth, hut fur- 

 nished ivith an ovate hollou\ and mostly lateral trans- 

 verse grooves. 



There are no fewer than one hundred and thirty-six 

 species in this genus, and wliich constitute an inexhausti- 

 ble mine of endless beauty and variety. 



The Ostreae may be divided into two classes: the first 

 is that which comprises the 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 outline, describing in their course the most 

 graceful expansion possible. It usually happens that the 

 Ostreae are inequivalve, that is, the degree of convexity of- 

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

 Ostrea Ziczac, Jacobaea, &c. &c. which invariably have 

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



