60 PALiEONTOLOGY OF NEW- YORK. 



valve, the muscular impression is subpentagonal, deeply bilobed below ; 

 the bases of the divisions rounded, and the sides slightly indented. The 

 area is narrow, incurved in the middle, and extends about two-thirds the 

 width of the shell. 



This species resembles Orthis carinata; but among numerous specimens col- 

 lected, none of them are more than two-thirds the length and breadth of the 

 specimens of that species known to me. The dorsal valve is not so gibbous as in 

 0. carinata, nor the sinus so well defined ; while the striae are much stronger upon 

 the surface of the .cast. Notwithstanding these differences, however, I am prepared 

 to find specimens which may show the unity of these forms as one species. 



This species occurs in the shaly sandstone and shale, and sometimes in semi- 

 calcareous bands, near Factoryville in Tioga county, along the Cayuta creek, at 

 Chemung-narrows, near Elmira, at Horseheads, and at Buck's quarry. It occurs 

 also in Allegany county at Philipsburgh, and near Leon and other places in 

 Cattaraugus county, New- York. 



Orthis iiiipressa. 



PLVTE VIII. 



OrthU impreiia : Hail, Gcol. Report Fourth District New-York, 1843, page 2G8, and flg. 2, p. 267. 

 Compare OrlhU tulliensu, Vasuxeh, Geol. Report Third District, p. 55. 



Shell rotund. Dorsal valve very gibbose, wider jjhan long, sinuate in 

 front : hinge-line about two-thirds the width of the shell. Ventral 

 valve moderately convex at the sides, somewhat flattened on the umbo, 

 with a broad undefined sinus which becomes deeper towards the front, 

 the margin of the shell being sometimes abruptly incurved : area of 

 moderate height, a little incurved at the beak. 



The surface is very finely and evenly striated, and the texture of the 

 shell is minutely punctate. 



The cast of the dorsal valve shows a strong, somewhat quadrilobate 

 muscular impression, limited by strong and widely diverging socket- 

 plates, with the vascular impressions somewhat narrow and extending 

 below it to the margin of the shell. The surface of the cast preserves fine 

 even striae. The cast of the ventral valve is broadly sinuate in the middle 

 below, with a triangular or subovate deeply bilobed muscular impression, 

 which is subject to considerable variation in form and proportions. 



