BIVALVE SHELLS. 237 



The specimen is from Filey Bridge. This is the o. diluviana 

 of Linnaeus, figured by Parkinson, III. PI. XV. Fig. 1; and by 

 Sowerby, Tab. 48, under the name o. Marshii. The recent o. 

 Sinensis is probably of the same species, which admits of many 

 varieties. Fig. 1, PI. X, displays the plaited edge of an oyster 

 of this family, from the upper shale. The great thickness of its edge, 

 with some differences in its shape, might entitle it to be regarded as 

 a distinct species from that last mentioned, had we not intermediate 

 specimens to connect them. It would seem, that in the younger 

 shells, such as No. 14, PI. IX, the plaits are few, and only two of 

 three of the laminae that compose them reach the margin, while a 

 succession of other laminae may be seen proceeding towards it, at 

 different distances ; whereas in the old shells, such as No. 1, PI. X, 

 the plaits are very numerous, and tbeir laminae have all, or almost 

 all, arrived at the margin, where they form a solid zigzag boi'der. 

 In the former, we find a protuberance, more or less broad, in the 

 region from whence the plaits diverge ; in the latter, this protuber- 

 ance covers most of the disk, appearing like another valve adhering 

 to the back of the shell. Further, the young specimen is somewhat 

 oblong oval, measuring about three inches long, and above two 

 broad ; but the old specimen is of a deltoid shape, having a pro- 

 jecting lobe on one side of the base, and measures above four inches 

 long, and nearly four in breadth, from the point of the lobe to the 

 opposite side. The growth of lobes often forms the difference 

 between old and young univalves, and the same curious fact seems 

 to occur here. The hinge of this shell is striated across. 



Fig. 2, PI. X, is a singular oyster from the Speeton or upper 

 shale, resembling the hoof of an ox, and having a narrow incurved 

 beak. The shell is smooth without, or marked slightly with lines of 

 growth. It has a deep oblong cavity within, extending from the 

 bejak to the base. The one side of this cavity is the flat part or base 



3 N 



