230 PALEONTOLOGY OF THE EUKEKA DISTRICT. 



Formation and localities. Lower portion of the Carboniferous Group 

 with the preceding, and also on the west slope of Richmond Mountain, 

 Eureka District, Nevada. 



Aviculopecten, 8p. f 



Two other species of the genus are shown by rather large-sized shells, 

 but their condition is such that the specific determination would be uncer- 

 tain, and with the genus so well represented they are left for more material 

 and future study to determine their specific relations. 



Genus STREBLOPTERIA McCoy. 



Streblopteria siniilis, n. s]>. 

 Plate viii, figs. 4, 4a-d, and plate xis, fig. 7 



Shell of medium size; outline, exclusive of the anterior ear, ovate, 

 general surface depressed convex. The two valves are very much alike in 

 convexity and form, except at the anterior ear and the slope beneath; the 

 right valve has a deep, sharply-cut byssal sinus beneath the ear, the ear 

 joining the body of the shell higher up towards the beak. The posterior 

 ear is either very short and small, or obsolete, being merged into the flat- 

 tened posterior umbonal slope. Hinge-line very short. Beaks small, the 

 sides sloping away at an angle of about 80. 



Surface of the larger shells smooth or with fine concentric striae and 

 lines of growth; a few very indistinct fine radii are present towards the lower 

 margin; the anterior ear of the right valve is marked by concentric lines 

 of growth and rather strong radiating costse; on the left valve the anterior 

 ear has only the concentric strise. On all the smaller shells the raised ra- 

 diating striae or fine costse are present in varying degrees of distinctness up 

 to well-marked striated forms that in surface characters apparently have no 

 specific relation to the large, smooth shells 



Dimensions: height of two examples, 3.75 cm and 1.75 cm ; breadth, 3.25 om 

 and 1.5 cm ; convexity of larger, 4 mm ; of smaller, about 2 mm . 



The peculiar obliquity of the valves, the deeply-cut sinus beneath the 

 anterior ear of the right valve, and the smooth surface relates this species 



