CONTRIBUTIONS TO PALEONTOLOGY. 33 



" ORTHIS ALSUS ( n. s.)." 



Shell somewhat semielliptical in outline : cardinal line equal to, 

 or more than two-thirds the greatest width, which is near or 

 just below the middle. Dorsal valve more convex than the ven- 

 tral, the greatest convexity being about the middle, sloping 

 almost equally to the sides and front and less rapidly to the 

 beak, while it is flattened or concave towards the cardinal an- 

 gles. The centre is marked by a sinus from the beak, which 

 gradually expands to the base of the shell. Yentral valve de- 

 pressed 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 striae, 

 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 O. ohlata of the Lower 

 Helderberg, 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 the 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 valve ; but it is almost al- 

 ways 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 

 interior, with impressions of the exterior surface, usually as separated 

 valves, and rarely with the impression of both valves in their natural rela- 

 tion. 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 Helderberg mountains in Albany county, 

 and also in Schoharie county. 



[Senate, No. 115.] 5 



