NATURAL SCIENCES OF PHILADELPHIA. 321 



are really distinct specificallj^, the 31. Casei being marked by 

 very numerous alternating larger and smaller radiating strife, 

 ■while M. alata is ornamented with large, strong radiating ribs. 

 M. Casei also differs in having its unibonal slopes distinctl}^ an- 

 gular, instead of broadly and evenly rounded, as in the species 

 under consideration ; while its ventral margin is angular in out- 

 line, at the termination of the umbonal ridge, instead of being 

 rounded. Its marks of growth also show that the extremity of its 

 wing was ratlier acutely pointed, instead of being a little rounded. 

 Locality and position. — Clinton County, Ohio, in upper part of 

 the Cincinnati group of the Lower Silurian. Mr. James's collec- 

 tion. 



MEGAMBONIA JAMESI, Meek. 



Megambonia? Spinneri? James, 1871. Cat. Fossils Cincinnati Group, p. 12. 

 (Not if. Spinneri, Hall.) 



Shell attaining a rather large size, a little obliquely subovate 

 in general form, rather convex, the most gibbous part being some- 

 what above and in front of the middle, more or less abruptly 

 cuneate posteriorly and below ; basal outline regularly rounded ; 

 posterior margin rounding into the base, and ascending with a 

 convex curve and forward inclination to the posterior extremity 

 of the hinge, which is not in the slightest degree alate ; anterior 

 margin rounding into the base below, and slightly sinuous under 

 the lobe-like protuberance, or rudimentary wing above, which is 

 convex, slightly more prominent than the margin below, and de- 

 fined from the swell of the umbonal regions on each side, by an 

 oblique sulcus extending to the hinge margin in front of each 

 beak ; hinge equalling about two-thirds the antero-posterior dia- 

 meter of the valves ; beaks rather prominent, or rising distinctly 

 above the hinge line, but slightly oblique, and distinctly incurved; 

 umbonal slopes broadly rounded; longer axis of the valves mode- 

 rately oblique to the hinge line. Surface ornamented by very 

 regular, rounded, simple, and depressed radiating costje, a little 

 wider than the furrows between, and numbering about five in a 

 space of 0.30 inch, near the middle of the lower margin. 



Height, about 2.05 inches ; antero-posterior diameter, 2.1 6 inches; 

 convexity, 1.50 inch. 



The only specimen of this species I have seen is a cast of the 

 exterior, with portions of the ventral and anterior ventral mar- 

 18t2.] 



