96 PALAEONTOLOGY. 



angle of about 95°; surface ornaineuted by rather obscure, ixnequal, radi- 

 ating costse, and numerous, very regular, extremely fine, concentiic striae, 

 scarcely visible without the aid of a magnifier ; costal not defined on the 

 ears, or usually on the adjacent lateral mai-gins, and most frequently 

 arranged Avith from one to thnio smaller ones between each two of the 

 largest, the middle one of the three smaller being sometimes a little larger 

 than the smallest one on each side. Right valve as convex as the other, or 

 sometimes more compressed, but without traces of radiating costse or striae ; 

 ears of the same size, and nearly of the same form as in the other valve, 

 excepting that the anterior one is defined by a deeper marginal notch. 



Length, or antero-posterior diameter of a large specimen, 1.70 inches; 

 height of the same, 1.73 inches ; convexity of left valve, 0.25 inch. 



I amnot positiveli/ sure that the two valves described above belong to 

 the same species, not having seen them in any instance united. The fact, 

 however, that they occur together in the same matrix, and have the same 

 form, and do not differ very greatly in size, while the smooth specimens are 

 all right valves, and the costated ones left valves, leaves httle room for 

 doubting that they are the opposite valves of the same species. The most 

 obvious characters of this shell are its nearly circular foim, subequal ears, 

 and the very fine, regular, concentric striae, and unequal radiating costae 

 of its left valve. I know of no other species nearly enough allied to render 

 a comparison necessary. 



The form to which the costate valves belong may be considered the 

 type of the species, in case the smooth valves should prove to belong to a 

 different shell. 



Locality and jjosition. — In light-colored Carboniferovis limestone, at 

 latitude 39° 33' N., longitude 115° 12' W. ; Colonel Simpson's collection. 



AVICULOPECTEN OCCIDANEUS, Meek. 

 Plate 12, figs. 13, 13 a, 13 b. 



Shell small, usually very little oblique, broad ovate-suborbicular in out- 

 line exclusive of the ears ; hinge shorter than the antero-posterior diameter; 

 pallial mai-gin more or less regularly rounded, but generally a little more 

 prominent behind the middle than before. Left val\e moderately convex; 



