SUBCARBONIFEKOUS PEHIOD. 91 



separated by very narrow interspaces, becoming smaller toward, and finally 

 disapjjearing upon, the postero-lateral regions. 



Only the ventral valve of this species is contained in the collections ; 

 but that agrees in all essential characters with S. peculiaris Shumard, the 

 type-specimens of which species were obtained from the Kinderhook for- 

 mation of the Subcarboniferous group in Missouri. The principal variations 

 which it shows from the typical forms as described and figured by Dr. 

 Shumard are the incipient plications of the mesial sinus and the somewhat 

 greater rounding- of the postero-lateral angles. Possibly more perfect 

 examples may show other differences also, but the characters so far observed 

 do not seem to warrant a specific separation from S. j)eculiaris. 



Position and locality. — Strata of the Subcarboniferous period ; Mountain 

 Spring", old Mormon road, Nevada. 



Genus SPIRIGEKA U'Orbisny, 1847. 

 Spirigera monticola White. 



Plate V, fig. 11, a, b, c, and d. 



Spirigera monticola White, 1874, Exp. & Surv. west 100th Merid., Pi'eliin. Eep. Invert. 

 Foss., IG. 



Shell subelliptical or subtetrahedi-al in outline, always wider than long, 

 widest at, or a little forward of, the middle, moderately gibbous ; valves 

 almost equ.ally capacious ; postero-lateral margins in old shells thickened, 

 but in younger ones the whole margin is more or less sharp ; front margin 

 only slightly sinuous in very young shells, but it is very deeply sinuous in 

 some old ones. 



Ventral valve broadly convex from side to side, regularly arching from 

 beak to front ; beak moderately prominent and slightly incurved ; foramen, 

 as usual, nearly round, rather small ; mesial sinus moderately narrow, 

 scarcely apparent in young sheWs, but in some old ones becoming very deep 

 at the front, where the margin is much prolonged upward to follow the fold 

 of the other valve. 



Dorsal valve gibbous in the umbonal region, prominent along the mid- 

 dle, from which the sides slope away by gentle convexity to the lateral 

 margins ; mesial fold rather narrow, and in some e:iamples not well defined. 



