68 PALEONTOLOGY OF THE EUREKA DISTRICT. 
rounded in front, widest one-third the length from the front, thence round- 
ing and narrowing gently to the beak, which is rather broadly rounded ; 
area unknown. ‘The dorsal valve is transversely ovate to subcircular, beak 
emarginate. 
The interior of the ventral valve is unknown; that of the dorsal valve 
shows a central median depression divided centrally by a narrow raised line; 
towards the beak the depression is broader, narrowing midway, and dividing 
before reaching the center of the shell; a large muscular scar occurs each side 
of the central depression, and also a depressed line a little beyond them that 
runs out obliquely from the beak. There appears to be a muscular scar on 
each side of the center of the median depression just at its bifurcation. No 
other scars could be detected. 
In external form and character this species is allied to Obolella gemma, 
and 0. chromatica of Billings. The arrangement of the muscular scars in the 
smaller valve is peculiar and the shell is also thinner than in the above 
species. 
The generic reference is made with a query as in comparing it with the 
dorsal valve of Obolella sagittalis Salter, as illustrated by Davidson, we find 
a marked difference and would scarcely refer the two to the same genus. 
In a comparison with American species the reference would be to Obolella, 
as the interior of the dorsal valve has not been given in the typical species, 
and that of Obolella polita has little if any more than a strong specific varia- 
tion from that of O.? ambigua. 
Mr. Davidson, in speaking of the species arranged under Obolella, says 
that the diagnosis of the genus will require modification before it can com- 
prise O. chromatica (the type) O.? polita, O.? Salteri, O. desiderata, ete. From 
our limited study and comparisons it does not appear that all these can be 
united under one genus—0O. polita and O. ambigua presenting one type, O. 
gemma and O. desquamata another, and O. sagittalis Salter, still another among 
the species now referred to the genus. 
Formation and locality—Pogonip Group. Base of Silurian. East slope 
of ridge east of the Hamburg mine, Eureka District, Nevada. 
