BIVALVE SHELLS 223 



some are flat and broad in the form of the letter D ; and some 

 approach the shape of the trigonia; yet, as some of all these forms 

 are marked with the same waved or zigzag ribs, crossed by the same 

 kind of zones, we can scarcely consider them as any thing more than 

 different varieties of the same species. By adopting an opposite 

 course, we might multiply species ad infinitum; so many shades of 

 difference being discernible. Sowerby's plagiostoma ovalis, p. cardii- 

 formis, p. obscura, and even p. pectinoides, may all be varieties of p. 

 rigida. Nay, we may perhaps add p. punctata, and p. gigantea. The 

 former differs from p. rigida, chiefly in being striated more finely, 

 and marked with fainter circles ; which may be accounted for by a 

 difference of the matrix, as well as some accidental variation in the 

 shell. In one of our specimens, of the donax shape, i)art of the 

 shell is nearly or entirely smooth ; part is striated with rows of points, 

 as in the p. punctata ; and part of it is marked with the deep waved 

 lines of jo. rigida; according as the shell has been more or less ex- 

 posed, the difference having apparently been produced by friction. 



P. gigantea seems to have no better claim to rank as a distinct 

 species. We have a deltoid specimen from the grey limestone, aboiit 

 four inches in diameter, corresponding with Mr. Sowerby's speci- 

 men. Its surface is mostly smooth, or marked with faint striae ; but 

 the faintness of its markings may be viewed as the result of age, or 

 of the decomposition or removal of the outer part of the shell ; 

 for in the front, where it has been least exposed, we find some 

 strongly marked ribs, somewhat waved. 



P. spinosa of Sowerby, must be regarded as a distinct species ; 

 not only on account of its spines, M'hich are sometimes wanting, but 

 also on account of the coarseness of its ribs or furrows. Among our 

 specimens is one very coarsely ribbed, from the oolite, as large as 

 that last mentioned, but with scarcely any vestiges of spines. It is 

 more of a deltoid shape than Mr. Sowerby's specimen, and the absence 



