ORISKANY SANDSTONE. 491 



The interior of the ventral valve shows strong dental lamellae, which 

 are curved backwards beneath the area, and do not extend so low on the 

 shell as its lower margin. Muscular area not defined in the specimen 

 examined. 



This species resembles the S. cycloptera of the Lower Helderberg group; and it 

 fliay be only a variety of form, resulting from a change of conditions in the sedi- 

 ment. The proportions are usually somewhat different, and the area is higher than 

 in that species. 



Fig. 8 a, 6. Ventral and dorsal views of a specimen of ordinary size. 



Fig. 8 c. Front view of a specimen which is less gibbous than usual. 



Fig. 8 rf. Profile view of a more gibbous specimen. ( The dorsal valve is represented as too 

 convex.) 



Fig. 8 e. An enlargement of the surface, showing the fine concentric and radiating strise. 

 Geological position and locality. In the Oriskany sandstone : Cumberland, Md. 



Spirifer cniiiberlandla>. 



Plate XCVI. Fio. 9 a - g. . 

 Spirifer cumberlandiet : Hall, Regents' Report of 1856, p. 63; Palseozoic Fossils, 1857, p. 23. 



Shell broadly semicircular ; valves moderately and nearly equally con- 

 vex. Ventral valve regularly convex ; mesial sinus narrow, shallow, 

 and flat in the middle : beak gently incurved, and projecting slightly 

 beyond the hinge-line ; area broad, nearly flat, parallel with the axis 

 of the shell ; foramen somewhat large, often partially or entirely closed. 

 Dorsal valve having a narrow flattened mesial fold, convex in the 

 middle, and flattened towards the extremities : beak scarcely incurved, 

 and nearly in the same plane with the cardinal margin ; hinge-line 

 straight ; extremities extended. 



SuKFACE marked by fourteen to seventeen simple rounded costje, which 

 are crossed by concentric elevated lines or lamellaB. 

 The muscular impressions in the ventral valve are well defined and 



distinctly striated longitudinally, with a broad median ridge, which, in 



its upper part, is marked by a narrow central crest. The dental lamellsa 



