33 



rounded ; front margin in general broadly convex, sometimes with a small 

 .space in the middle nearly straight. Dorsal valve of a medium convexity, 

 most ebvated about the middle; the outline forming a uniform arch 

 from the depressed beak to the front margin ; the slope from the umbo 

 towards the cardinal angles, gently concave ; sometimes a barely percept- 

 ible mesial depression, commencing in a point at the beak, and becoming 

 obsolete at one-half or two-thirds the length ; area 'small, lying in the 

 plane of the lateral margins; beak minute, forming a small triangular 

 projection, rising scarcely one-fourth of a line above the edge of the area, 

 the point not incurved over, but situated in the plane of the area. Ven- 

 tral valve moderately convex, most elevated at between one-fourth and 

 one-third the length from the beak, thence descending with a somewhat 

 flat or gently convex slope, to the front and sides, and with a more sudden 

 and somewhat concave slope to the hinge line and cardinal angles ; the 

 umbo small, prominent, neatly defined, terminating in a small rounded 

 beak, which is incurved so as to overhang the edge of the area, either not 

 ^t all or scarcely one-tenth of a line ; area triangular, about one-fourth 

 larger than that of the dorsal valve, forming an angle of about 105? with 

 the plane of the lateral margin. The foramen not observed, but appears 

 to be wider than high. On looking at the dorsal valve in a direction 

 perpendicular to the plane of the vshcll, the small rounded umbo of the 

 Tentral valve can be seen rising about one-third of a line above the dorsal 

 'beak. 



IG. — Dorsal view. 



Surface with small sub-angular radiating ridges, of nearly a uniform 

 size, from eight to ten in the width of three lines, increasing by bifurca- 

 tion, strongly curved outwards to the upper part of the sides and cardinal 

 angles, the intervening grooves sub-angular in the bottom, and equal to 



