9^ PALEONTOLOGY. 



angle of about 95°; surface ornamented by rather obscure, unequal, radi- 

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

 scarcely visible without the aid of a magnifier ; co^se not defined on the 

 ears, or usually on the adjacent lateral margins, and most frequently 

 arranged with from one to thn^e 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 am not positively 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 little room for 

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

 obvious characters of this shell are its nearly circular form, 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 t-o 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 position. — In light-colored Carboniferous limestone, at 

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



AVICULOPECTEN OCClDANEUS, Meek. 

 Plate 12, figs. 13, 13 a, 13 6. 



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

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

 pallial. margin more or less regularly rounded, but generally a little more 

 prominent behind the middle than before. Left valve moderately convex; 



