No. 109.] 63 



the lateral extremities; beak little elevated and scarcely in- 

 curved ; mesial sinus shallow, and flat in the middle : dorsal 

 valve depressed-convex in the middle and flattened laterally, 

 often a little concave towards the extremities ; mesial fold dis- 

 tinctly defined, equal in width to the two adjoining costse, some- 

 what flattened in the centre ; beak little elevated above the 

 hinge-line, and scarcely incurved; area moderate; foramen 

 somewhat large, often partially closed. Surface marked by ten 

 to fourteen simple rounded and moderately elevated costse ; 

 concentrically marked by imbricating lamellae, which are usual- 

 ly almost obsolete or obliterated when the shell is silicified. 



This differs from the preceding species in being a smaller and more 

 delicate shell, with the extremities more distinctly mucronate. The surface 

 is less strongly marked with concentric lamellse. 



Geological position and locality. Oriskany sandstone, Cumberland, Md. 



SpIRIFER CUMBEELANDl^ ( 11. S.). 



Pal. N.Y. Vol.iii, pi. 100, f.4. 



Shell broadly semicircular; valves moderately and nearly equally 

 convex : ventral valve regularly convex ; mesial sinus narrow, 

 shallow, and flat in the middle; beak gently incurved, and 

 projecting slightly beyond the hinge-line : dorsal valve having 

 a narrow flattened mesial fold, with a faint depression down the 

 centre ; beak scarcely incurved, and nearly in the same plane 

 with the cardinal margin ; hinge-line straight ; extremities ex- 

 tended ; area broad, nearly flat, parallel with the axis of the 

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

 Surface marked by from fourteen to seventeen simple rounded 

 costse, which are crossed by concentric elevated lines or lamellse. 



In general form, this shell resembles S. mucronatus, but is conspicuous- 

 ly distinct in its wider area. It is usually broader, with the mesial sinus 

 and elevation narrower. 



Geological position and locality. Ojiskany sandstone, Cumberland, Md, 



