ORTHTDES OF THE UPPER HELDERBERG ROCKS. 87 



beak, which gradually expands to the base of the shell. Ventral valve 

 depressed -convex, and nearly flat in the middle of the lower half: 

 beak small and neatly defined, projecting slightly beyond the area-line ; 

 area narrow sublinear, a little inclined ; foramen partially filled by the 

 strong cardinal process of the opposite valve. Area of the dorsal valve 

 comparatively large, equal to two-thirds the width of the area of the 

 ventral valve, flat or lying in the plane of the margins of the valve. 

 Surface marked by fine, somewhat unequal, bifurcating stri£B, about 

 eight or nine in one-fifth of an inch, much curved on the upper lateral 

 margins. 



The casts of this species are of the character of 0. oblata of the Lower Helder- 

 berg, and others in the Hamilton group ; but the hinge-line and area are much 

 more extended, distinguishing this species from any in those strata with similar 

 muscular impressions. 



The cast of the dorsal valve, when the impression of the area is not preserved, 

 is not readily distinguishable from some of those mentioned ; though in well- 

 preserved specimens, the depression down ther centre, and also across the middle 

 of the muscular impression, is more strongly marked than in those species. 



The muscular impression of the ventral valve is proportionally small, being 

 usually less than half the length of the vftlve ; but it is almost always strongly 

 defined, which, with the long hinge-line, are marked features. 



This species occurs in the Schoharie grit in the condition of casts of the inte- 

 rior, with impressions of the exterior surface, usually as separated valves, and 

 rarely with the impression of both valves in their natural relation. From one of 

 these specimens, a cast in sulphur has given the exterior form and proportions of 

 the fossil, as well as the surface markings ; and a single dorsal valve, retaining 

 the shell, has been found among the entire collection from this rock. 



Geological formation and locality. In the Schoharie grit : at Clarksville, Knox, 

 and other places in the Helderbcrg mountains in Albany county, and also in 

 Schoharie. 



Oithis niitis ( n. s.). 



PLATE V. 

 Shell suborbicular, of moderate convexity ; area about half the width of 

 the shell ; length and width about as four to five ; cardinal extremities 

 rounded. Surface somewhat coarsely striated. 



