BIVALVIA. 49 



• 



Shell small, ovate, equilateral, tumid, and closed, costated, central portion with 

 7-9 angulated ribs, becoming obsolete or disappearing on the sides ; hinge margin 

 straight, with a trigonal and concave pit for the cartilage. 



Hei(/lit, -^. Length, -jV- I>epth of united Valves, yV of an inch. 



Locality. Cor. Crag, Sutton. 



This pretty little shell is very abundant in the Coralline Crag, at the above 

 locality. 



It appears to be quite distinct from the preceding, and differs in several 

 characters, and there is no species recent or fossil known to me to which it can be 

 assigned. 



It has about seven angularly formed costse, which occupy the central portion 

 of the dorsal area, beyond these are faint traces of striae, and in well preserved 

 specimens the shell is semi-transparent, rendering these ribs visible in the interior, and 

 giving about half a dozen rough crenulations to the ventral margin of the shell. 



The ligamental area is large, and the central pit diverges from the umbo under an 

 angle of about 80°, muscular impression subcentral and ovate. It differs from any of 

 the specimens of the preceding species of the same size, in being thicker and more 

 regularly ovate, the ventral margin is more pointed, the sides are less straight, 

 while the costse are more prominent and distinct. I have not seen L. sidculus, 

 Leach and Loven, but the descriptions do not accord with our Crag shell, as it wants 

 the " mediis binis verticalibus," mentioned by the latter author, but which character 

 may be seen in specimens of L. subauriculata. 



Pinna.* Linnceus. 



Pinna. Arist. Aldrov. List, Linn. Lam., ^c. 



Pennakia. Browne, 1756. 



Chim.era et Chim^rodekma. Poli., 1795. 



Perna. Adans, 1757. 



OxYSMA? Rafinesque, 1819. 



CURVULA. Id. 



Atrina. Gray, 18J0. 



Generic Character. Shell equivalved, inequilateral, oblique, triangular or wedge- 

 shaped, generally thin and fragile ; umbones terminal, hinge rectilinear, without teeth ; 

 anterior margin sinuated and slightly gaping for the passage of a byssus ; posterior 

 truncated. Impression by the mantle entire ; hgament internal. 



Animal triangular, in conformity with the shape of the shell; its mantle open or 

 disconnected on all sides, except the dorsal edge, while its lobes line the whole 

 interior of the valves; the lobes are ornamented with a double row of tentacular 



* Etym. TTiiia, Arist., a kind of Pearl Oyster. 



