230 PALEONTOLOGY OF THE EUREKA 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, sp.? 
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 similis, n. sp. 
Plate viii, figs. 4, 4a-d, and plate xix, 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 striz 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 coste; on the left valve the anterior 
ear has only the concentric striz. On all the smaller shells the raised ra- 
diating striz or fine costz 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™ and 1.75"; breadth, 3.25 
and 1.5°%™; convexity of larger, 4""; of smaller, about 2™™. 
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 
