12 MOLLUSCA FROM THE CRAG. 



A)wmia striata, J. Sow., Min. Conch., t. 425, differs from our shell, in hiiving more 

 numerons and much finer striae 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 

 Conchologists, 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 tiic Crag species, 

 to assign them to what is Ijclieved to be identity of form or correspondence with those 

 shells which are found in recent seas. 



OsTREA, Linn. 1758.* 



OsTREA. Lister, 1686. 



OsTR.EA. Befranee. Swains. Reeve and Callow. 



OsTKEUM. Ruwpk. 1705. Adanson, 1/57. 



Mya. Scopoli, 1777. 



Pelouis and Peloiuderma. Poll. 1791. 



Dexdostejea. Swaim. 1840. 



Gen. Char. Shell attached by a part of the larger or lower valve, generally thick 

 and strong, lamellated or foliated, variously shaped, irregular, inequivalvcd, inequi- 

 lateral ; upper or free valve flat or slightly concave ; under or adherent one convex, 

 sometimes strongly marked with radiating, lamellated costae ; hinge without teeth, 

 ligament lodged in a linear depression in each valve scmiexternal. 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. ortTfteoi, a fish, (ourtoi) ? 



