238 ORGANIC REMAINS. 



of the hoof; the other is a thick ridge running along the inside of the 

 curved part of the hoof. Above this thick ridge rises a thin lobe or 

 margin, which seems to have been continued round the base of the 

 shell, but is always found imperfect. We have not seen a specimen 

 with the smaller valve, or cover. From its remarkable shape, we may 

 call this shell o. unguis. Its curved beak resembles that oi a gryphrsa. 



In the same shale, we find a large, massive, and very deep 

 oyster, of a deltoid or subtriangular shape ; with a very rough knotty 

 exterior, but smooth within. Its beak is incurved, like that of 

 o. unguis ; but it is rough and irregular in its appearance, with trans- 

 verse coarse markings, which are more or less continued on the disk, 

 and which are so distorted, that they can scarcely be called either 

 striae or folds. The inside displays a large and deep subtriangular 

 cavity. The cover, or lesser valve, is of course subtriangular; and it 

 is concave, having a considerable hollow in the middle. Both valves 

 are very thick ; and in both, especially in the smaller valve, the large 

 muscular impression is deeply marked. Beside it, in the larger 

 valve of our best specimen, is a knob containing a pearl. This specimen 

 is five inches long, four broad, and nearly three in depth. It weighs 

 twenty-six ounces. This oyster, like o. unguis, appears to be a non- 

 descript; and, from its large cavity, we may term it o. capax. 



Fig. 16, PI. IX, is the o. gregaria of Sowerby, Tab. Ill; called 

 by some o. frons, and not unlike the o. pectinata of Lamarck. It is 

 found in the oolite, and in the calcareo-ferruginous sandstone. The 

 shell is finely plaited, and the margin displays zigzag markings, as 

 in o. diluviana. It has often a bend near the beaks; where also we 

 sometimes find a lobe, or excrescence, on one side. The shells of 

 this family are often found in clusters, adhering together. A thin 

 parasitical shell, with obtuse plaits in the margin, like the o. folium, 

 occurs also in the calcareous sandstone. It is not uncommon to find 

 even large oysters adhering together, particularly in the upper shale. 



