FOSSILS OF THE LOWER SILURIAN. 



71 



The dorsal valve, fig. 36, shows a narrow median ridge with a pair of 

 minute, elongate, oval scars at its front termination (a, a) ; posterior to these 

 and between the central ridge and a low ridge extending from the beak 

 obliquely outward and forward a larger, reniform scar occurs on each side 

 (p, a) ; outside of the narrow lateral ridges and on the posterior lateral por- 

 tion of the shell another minute scar is seen on each side (a, d). None of 

 the specimens show a scar beneath the beak. 



A comparison with the figures of Siphonotreta fissa Kutorga 6 shows a 

 strong generic relation, judging from the external characters; specifically 

 they have several points in common. 



The species occurs in the passage beds at the base of the Pogonip 

 Group in the same band of layers carrying Acrotreta gemma, Lingulepis, 

 Discina, Orthis, Camarella calcifera, Ptychoparia Oweni, Dicellocephalus 

 finalis, Illcenurus Eurekensis, Amphion, Barrandia McCoyi, etc. 



Formation and locality. Lower portion of Pogonip Group. East slope 

 of ridge east of the Hamburg mine Canon, Eureka District, Nevada. 



Genus STROPHOMENA Rafinesque. 



Strophomena Nemea, H. & W. 



Strophomena Nemea, Hall & Whitfield, 1877. Geol. Expl. Fortieth Par., vol. iv, p. 233, 

 pi. i, fig. 15. 



This species occurs in the upper beds of the Pogonip Group at Lone 

 Mountain, 18 miles northwest of Eureka, at the same horizon on McCoy's 

 Ridge, and a little lower down in the strata on the west slope of Secret 

 Canon road Canon, Eureka District, Nevada. 



The original type of the species is from Pogonip Mountain, White 

 Pine District, Nevada. 



6 Uber die BracMopoden-Familie der Siphonotretsese. Verhandl. der Kuss. Kais. Mineral. Gesell- 

 schaft, 1848, p. 250, pi. vii, figs. 5a-c. 



