so PALAEONTOLOGY OF NEW- YORK. 



valvo 18 also more gibbous, and the separated valves are conspicuously deeper 

 than in 0. vanuxcmi. 



This form is comparatively rare, occurring in few localities and having a li- 

 mited range, while the other has a wide geographical and great vertical range. 

 In one locality, great numbers of tliis species have been found limited to a ver- 

 tical range of a few inches ; while the other species occurs in the beds above 

 and below at the same place, with scarcely an individual mingled among these. 



Geological formation and locality. In the shales of the Hamilton group, at 

 Eighteen-mile creek, Erie county ; on the shores of Canandaigiia lake, Ontario 

 county, New-York ; and Cumberland, Maryland. 



Ortliis peiH'lope. 



PLATE VI. 

 OrlhU pentlope : }1\ll, Thirteenth Report on the State Cabinet, 1800, p. 79. 



Shell large, oblate, proportions of length and breadth usually as four to 

 five, plano-convex; hinge-line about two-fifths the width of the shell; 

 cardinal extremities regularly rounded. Dorsal valve regularly convex, 

 the greatest convexity about the centre, with a very slight mesial de- 

 pression or flattening along the centre : beak small, rising but little 

 beyond the general outline of the shell ; area little more than half aa 

 wide as the ventral area, not incurved, and lying nearly in a plane 

 with the anterior margin of the valve. Ventral valve depressed-convex 

 above, sometimes a little gibbous towards the umbo, flat or often con- 

 cave in the middle and below, the front without sinuosity : area in 

 the larger specimens nearly one-eighth of an inch in widtli ; foramen 

 broad, twice as wide as high, and nearly filled by the strong cardinal 

 process of the opposite valve ; ventral beak obtusely pointed and 

 scarcely incurved. 



Surface marked by fine radiating bifurcating stria?, which are strongly 



— ■''?bed upwards near the cardinal extremities, and crossed by fine 

 pomtecl I. 



- . - . • jilf 3> giving a slightly rugose appearance in well-preserved 



., ., , beside these are closely arranged lamellose lines of 



the more gibbous, . 



,, k 1 h '"S strisD have often the appearance of being broken 



