No. ] 0-91] 47 



This species Bears a general resemblance to O. ohlaia of the Delthyrfs^ 

 shale ;. but it attains a larger size, is more ventrieose, and never so dis- 

 tinctly resupinate. The beaks are more prominent and incurved, that of 

 the dorsal valve curving beyond the hinge-line. The cardinal views of the 

 two species also differ : the hinge-line of the present species arches up- 

 wards on each side of the beaks, while in the other it is straight. The area 

 is also higher in the Oriskany species than in that from the Shaly limestone. 



In the Oriskany sandstone of New- York, this species usually occurs in 

 the form of casts. I have obtained fine specimens of the fossil entire, and 

 of the separate valves, from Cumberland, Md. 



Geological ■position and locality. Oriskany sandstone, Albany and 

 Schoharie counties; Cumberland ( Md.), etc. 



Lept^na concava (n,s.). 



Pal. N.Y. Vol. iii, pi. 18, f. 2. 



Shell concavo-convex, hemispherical : ventral valve regularly 

 convex ; nmbonial region prominent ; cardinal margin rounding 

 from the beak towards the lateral extremities : dorsal valve 

 deeply concave ; hinge-line less than the greatest width of the 

 shell ; area of ventral valve broad, that of dorsal valve linear ; 

 foramen triangular, nearly closed by a thick callosity. Surface 

 marked by very fine, close, radiating strige, each fifth or sixth 



• one a little more prominent than those between ; crossed by fine 

 regular concentric wrinkleSj.producing a beautiful subcancellate 

 appearance. 



Geological position and. locality. In the shaly limestone of the Hel- 

 derberg group, Albany county. 



Lept^na nucleata (n. s.). 



Pal. N.Y. VoLiii, pL94, f. 1. 



Shell semicircular : ventral valve extremely gibbous, abruptly 

 depressed or fl,attened towards the lateral extremities; beak 

 ( internal cast ) very abruptly incurved, and divided by a cen- 

 tral groove ( left by the mesial plate) which extends back nearly 

 to the centre of the valve : dorsal valve unknown ; hinge-line 



