131 



above the cardinal edge ; area varying in size, from less than one half to 

 nearly equal that of the ventral valve. 



Surface with fine rounded slightly crenulated radiating striae of different 

 sizes, the smaller coming in by implantation at varions distances from the 

 beak. In some specimens the striae are more nearly of one uniform size 

 than in others ; at the front margin there are usually four of the larger 

 and four or five of the smaller striae in the width of one line. When the 

 surface is perfectly preserved, it is seen to be beautifully cancellated by 

 fine apparently squamose striae, which are undulated slightly upward in 

 passing over the ridges. There appear to be from ten to twelve concen- 

 tric striae in the width of one line. 



Width of largest specimen collected 1 inch ; length 9 lines ; height of 

 ventral area 1 line. 



Both Conrad and Hall describe the two areas as being nearly equal. 

 In only one of our specimens is the dorsal area preserved, and in that it is 

 about one fourth the width of the ventral area. The specimen figured hi 

 the 1st vol. of the Palaeontology of New York has a width on the hinge- 

 line about equal to twice the length of the shell. In none of ours is the 

 length more than two thirds the width. Notwithstanding these differ- 

 ences, I venture to refer ours to Conrad's species, as in all other respects 

 they agree with the figures and descriptions. 



This shell is closely allied to S. pecten, and may yet be found to be con- 

 nected therewith by intermediate forms. On examining a large number 

 of good specimens of jS. pecten, I find that that species varies a good deal. 

 In some of the individuals the radiating striae are straight in all parts of 

 the shell. In others, those in the middle are straight, but those near the 

 cardinal angles more or less convex. The concentric striae also vary hi 

 fineness from ten to twenty in the width of one line. The curvature of 

 the ventral valve also varies, it being in some individuals gently convex 

 all over except near the cardinal angles, and hi others convex at the 

 umbo and concave all round. The proportions vary, the hinge-line being 

 often much extended and the shell narrowest towards the front, or it is 

 equal to or even a little less than the greatest width. The area of the 

 dorsal valve is sometimes nearly equal to that of the. ventral, but often 

 less than half the size. The ventral valve is always depressed convex 

 or nearly flat, but never concave except near the cardinal angles. It 

 almost always exhibits a mesial depression exactly like that of S. recta. 



Not only does S. pecten agree with S. recta in the form of the valves 

 and characters of the surface, but it varies in the same way, and the two 

 species must be regarded as not only congeneric, but even as most closely 

 allied, if not absolutely identical. The only difference exhibited by our 



