BIVALVE SHELLS. 22 1 



chiefly in the oolite; but it is also found in the ironstone beds, and 

 in the dogger that covers the alum rock. Our largest specimen 

 measures two and a half inches in the one direction, and near one and 

 a half in the other. In its general shape it is very like No. 6 of PI. VII. 



Here also we may introduce the shell figured in PI. IX. Fig. 15. 

 It is from the lowest shale, or the hard seams in that shale; and is 

 chiefly found in Robin Hood's Bay. It is shaped nearly like No. 8 

 ■of PI. VII ; but is much more gibbous, has more prominent beaks, 

 and has a deep depression at the place of the cartilage, besides that 

 at the truncated end ; whereas the corresponding depression in the 

 other shell is generally slight. It has several strong ribs passing 

 from the beaks towards the base, and these are moi'e or less crossed 

 by transverse concentric ridges, especially near the base, or rounded 

 margin. The transverse lines are most prominent in the older shells ; 

 and in these, as was remarked of No. ] 2, PL VII, the ribs do not 

 reach the base, but there is a margin marked only with the concentric 

 lines. The truncated end is more slightly marked than the rest of 

 the shell. Each of the slopes, viewed in front, has the apjjearance 

 of a heart-shaped leaf The number of the ribs varies considerably ; 

 and they are most numerous in the younger shells. Some shells, 

 similar in size and form, apparently without ribs, occur in the same 

 place ; but of these we have not procured any good specimen. 



Plagiostoma. Wry-mouth. Under a genus of this name, 

 Mr. Sowerby has classed a pretty numerous family of shells, which 

 Mr. Parkinson, with some hesitation, placed under the genus donax. 

 This kind of shell, like the donax, has the area straight, flat, and 

 lengthened; and the base rounded. Its grand discriminating charac- 

 ter is, that it has small ears, one on each side of the beaks, forming 

 a line that crosses the beaks obliquely; and has in the one valve a 

 triangular cleft or opening, at the beak, while the other valve is entire, 

 or nearly entire. This cleft is principally in the right valve. 



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