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 Obohlla 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 0. f anibigua. 



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 0. chromatica (the type) 0. f polita, 0. f Saltcri, 0. desiderata, etc. From 

 our limited study and comparisons it does not appear that all these can be 

 united under one genus 0. polita and 0. anibigua presenting one type, 0. 

 gemma and 0. desquamata another, and 0. 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. 



