OSTREA* 



Gryphaea; but as we are compelled thus to express our 

 dissent from so great authority, we feel called upon also to 

 explain our reasons for such dissent : to do this, we must 

 first mention the particular points upon which Lamarck 

 depends for his distinction between them ; these are, first, 

 the apparent regularity of the GrT/phcece; secondly, their 

 being scarcely, if at all, attached; and, thirdly, the gene- 

 rally large, involute, spiral umbo of the lower valve. To the 

 first we answer, that though the Gri/phcdce are in general 

 apparently more regular than the Ustrece^ they cannot be 

 considered as regular shells, and that they are moreover very 

 variable ; secondly, the Gryphaeae, as well as all other 

 Ostrece^ are attached by the umbo of the larger and con- 

 cave valve, and this particularly in the young state ; (in 

 which state, in fact, it is impossible to distinguish between 

 one and the other:) moreover, both become free, as they 

 increase in size ; and if the Gryphaeae are then apparently 

 more regular, it is because in their young state they have 

 lived in situations where they could only become attached 

 to small, regular objects, while the Ostreae, having lived in 

 more rugged and irregular situations, and necessarily re- 

 maining attached for a longer time, have partaken more of 

 the irregularity of their native situation : indeed there is 

 sufficient evidence that an oyster, when by any chance it 

 becomes attached to a small, smooth object, where it is 

 comparatively free, becomes also regular in the same de- 

 gree ; thirdly, we cannot approve of the term spiral^ as at 

 any time applicable to the umbo of the lower valve of the 

 Gryphaeae; when young it is not involute: and though La- 

 marck mentions the size of this, as one important distin- 

 guishing mark of his Genus, he gives the characters of 

 several species in which this part is small. 



Another circumstance, in which the Gryphaea is thought 

 to differ from Ostrea, has been dwelt upon by some ; an 

 obscure lobe or sulcus observable on the right side, parti- 

 cularly of the lower valve ; but this is far from being 

 distinct in some species. 



All the species of the Genus Ostrea, agree therefore 

 in being irregular inequivalve shells, of a laminated or 

 foliaceous structure, the lamina being composed as in 

 Pinna of perpendicular fibres : the points of the umbones 

 are not quite close together, sometimes far distant, and 

 they become still more distant, as well as very unequal, by 

 age. The lower valve is larger than the upper, concave 



