FAMILY 5. OSTRACEA. 167 



OSTREA, Linnaeus. 



Testa affixa, inaequivalvis, irregularis, umbonibus subdivaricatis, inaequa- 

 lissimis, incremento arese plus minusve distantibus ; valvis fibro- 

 laminaribus ; inferiore majore, concava ; superiore minore planius- 

 cula. Cardo edentulus, ligamento semi-interno area valvarum 

 superposito. Impressio muscularis subcentralis. 



It may be noticed in the course of our observations on the preceding 

 family, that the genus Ostrea of Linnaeus has been variously dismembered 

 by Bruguiere and succeeding writers. Originally it included some of the 

 byssiferous Mollusca, such as the Mallei, the Pectines and others, whose 

 shells differ not only in their mode of attachment, but in being of an 

 entirely different structure and composition. It is, however, somewhat 

 curious to find that there are two or three species which, although strictly 

 belonging to the genus Ostrea in its present dismembered form, were not 

 referred to it by Linnaeus ; Lamarck, who has not failed to notice this ap- 

 parent error of discrimination in the great author of the ' Systema Naturae,' 

 mentions his Mytili hyotis,frons and crista-galli, as examples of those which 

 should certainly have been placed with the Ostreae. The shells of the 

 Ostreae exhibit considerable varieties of growth ; in fact, there is no 

 genus of mollusks in which we find such a multiplicity of distorted 

 forms. As the animal increases in age it gradually recedes from the 

 base of its shell, and the dorsal area which is thus formed (vide PI. CXX. 

 fig. 2), as in the shell of Spondylus and others, is sometimes so con- 

 siderably extended as to become modified or distorted to almost any 

 situation in which it happens to be confined. These inconstant modi- 

 fications of growth have been seized upon, as is too frequently the 

 case, for the formation of new genera. The genus Gryphcea, for exam- 

 ple, was proposed by Lamarck for the purpose of distinguishing those 



