90 [Assembly 



that species the extremities are usually rounded, and the shell a little 

 rounded below. 



Geological formation and locality. In the calcareous shales of the age 

 of the Hudson-river group : near Cincinnati, jDhio. Collection of S. T. 

 Carly. 



ORTHIS CLYTIE (n. s.). 



Shell larger than medium size, semielliptical, the length and 

 breadth about as 4 to 5, or as 7 to 9, plano-convex or concavo- 

 convex : hinge-line less than the greatest width of the shell ) 

 cardinal extremities rounded. Dorsal valve flat or concave, 

 with a longitudinal depression along the centre ; cardinal area 

 linear, not incurved. Ventral valve moderately convex, some- 

 times more elevated or subangular along the middle towards 

 the beak : area narrow, longitudinally striated, and extending 

 little more than two-thirds the width of the shell ; foramen 

 wide ; beak scarcely incurved. Muscular impression deeply 

 divided ; in the middle of the inner, division of each lobe, more 

 deeply impressed, and extending two-thirds the length of the 

 shell from the beak ; the outer portions flabellate, and mar- 

 gined by an elevated ridge ; the whole interior surface granu- 

 lose, and the visceral area margined by a thickened elevation, 

 which becomes, in old shells, a defined ridge. The strias are 

 shown along the inner margin. 



In the dorsal valve the inner surface is finely striato-granulose, 

 and outside of the submarginal ridge the striae are stronger. 

 The muscular impression is comparatively small, distinctly 

 circumscribed, with central ridge, thin prominent cardinal 

 process, and strongly defined dental fossets and corresponding 

 crural processes. 



The surface is finely and somewhat evenly striated ; the striae 

 on the ventral valve often more prominent than those on the 

 dorsal valve. 



This is a very well-marked and distinct species, which, in the deeply 

 bifid muscular impression, differs from any other species known to me 

 in this group of strata; while in other features, it is readily distin- 

 guished. 



