288 PALuEONTOLOGY. 



Genus AVICULOPECTEN McCoy. 



AVICULOPECTEN (EUMICROTIS ?) AUGUSTENSIS D. Sp. 

 Plate VII, fiRS. 14-16. 



Shell small, broadly ovate, higher than wide ; length of the hinge equal 

 to about half the height of the shell, the anterior side straight and forming 

 two-thirds of the entire length, posterior side shghtly declining from the 

 beak; left valve moderately convex, most prominent on the umbone; beak 

 small, obtusely pointed and but slightly projecting beyond the hinge-line; 

 auriculations depressed, but not distinctly separated from the body of the 

 shell, the anterior one of moderate size, posterior one quite small. Surface of 

 the valve marked by simple rounded ribs of equal strength, except on the 

 left auriculation, where they are finer, somewhat corrugated, and strongly 

 curved upward to the margin. Right valve flat or very slightly convex; 

 beak depressed and not extending beyond the hinge-line; ears much more 

 distinctly marked than on the left valve, the lines separating them from the 

 body of the shell, strong, nearly straight, and rapidly diverging from the 

 beak, inclosing an angle of about ninety degrees; anterior auriculation 

 large, rounding inward from the extremity. B}'ssal notch more than a third 

 as deep as the length of the ear, broad and rounded at the bottom. Surface- 

 markings similar to those of the opposite valve. 



The specimens from which the description is taken are slightly exfoliated 

 and do not present the natural surface-markings; but another fragment 

 presenting a weathered surface shows concentric striae, which are strongly 

 vaulted in crossing the radii, but not distinctly marked in the depressions. 



The species has much resemblance, especially the left valve, to many 

 of those referred by Mr. Meek to his genus Eumicrotis, in general form and 

 surface-markings, but differs strongly in the large anterior auriculation and 

 byssal notch of the right valve, corresponding in this respect more nearly 

 with Avicidopeden, and wu are undecided as to which genus they ought 

 properly to be referred. 



Formation and locality. — In limestone referred to the Jurassic, Shoshone 

 Springs, Augusta Mountains. Collected by S. F. Emmons, esq. 



