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12 MOLLUSCA FROM THE CRAG. 
Anomia striata, J. Sow., Min. Conch., t. 425, differs from our shell, in having more 
numerous and much finer strize without the roughness of the Crag specimens, and is in 
all probability distinct; the umbo of the Eocene shell extends to the edge, or very 
nearly so, and was, probably, not so thick in substance. 
These, as before remarked, have been separated into different species by recent 
Conchoilogists, but their correct specific distinction cannot be expected in fossils, 
when the recent forms are so perplexing as to defy determination, or at least to pro- 
duce great diversity of opinion; it is, therefore, only attempted with the Crag species, 
to assign them to what is believed to be identity of form or correspondence with those 
‘shells which are found in recent seas. 
OstrREA, Linn. 1758.* 
OstreA. Lister, 1686. 
OstrmA. Defrance. Swains. Reeve and Catlow. 
Ostreum. Rumph. 1705. <Adanson, 1757. 
Mya. WScopoli, 1777, 
Prxorts and PetortpErMA. Poli. 1791. 
DenpostRma. Swains. 1840. 
Gen. Char. Shell attached by a part of the larger or lower valve, generally thick 
and strong, lamellated or foliated, variously shaped, irregular, mequivalved, inequi- 
lateral; upper or free valve flat or slightly concave; under or adherent one convex, 
sometimes strongly marked with radiating, lamellated costee; hinge without teeth, 
ligament lodged in a linear depression in each valve semiexternal. Impression 
of the adductor muscle, large subcentral, that by the mantle entire, generally indis- 
tinct, and ill defined. 
The animal has the mantle disunited on all sides, with its edges bordered by 
short tentacular fringes ; foot obsolete. Sexes distinct. 
The shells of this genus have only one muscular impression, which is always a little 
inclined to the posterior side. These animals fix themselves by the exterior of the 
left valve, and the space upon the shell denoting the place of adherence is exceedingly 
variable in size, depending, probably, upon external causes; in some individuals, the 
greater part of the entire surface is employed, while in others, this place of attachment 
is scarcely to be discerned, and occupies only a small portion of the pointed umbo of 
the shell. The ligament may be considered as external, separating the valves by its 
contraction when the adductor is relaxed. This ligament takes its rise at the extreme 
* Etym. dorpeor, a fish, (ooreor) ? 
