No. 109.] 53 



wrinkles stronger, especially near the beak : they are, also, generally more 

 curved outwards near the hinge. The lateral margins of our shell arc like- 

 ■wise, in most specimens, more contracted below the extremities of the 

 hinge, than in the examples given by Barrande. 



Geological posilio/i and locality. Shaly limestone of the Lower Hel- 

 derberg, Albany and Schoharie counties. 



StROPHOIMENA ( StROPHODONTa) LEAVENWORTHANA (U.S.). 



Pal. N.Y. Vol. iii, pi. 21, f.5-7. 



Shell subsemicircular, about three-fourths as long as wide, con- 

 tracted below tlie extremities of the hinge; cardinal border 

 sloping slightly from the beak : ventral valve flattened in the 

 middle and cardinal margin, so as to form a semicircular inclined 

 plane ascending from the hinge to beyond the middle of the 

 shell ; the front and lateral margins abruptly inflected, giving 

 a deep concavity to the whole valve : dorsal valve flattened 

 or slightly concave in the umbonial and central regions, very 

 convex towards the front and lateral margins : hinge-line equal 

 to the greatest width of the shell, crenulated ; area linear, ver- 

 tically striated ; foramen small, triangular, closed in full-grown 

 individuals. Surface marked by fine, obscure, closely arranged, 

 radiating striae, crossed on the depressed part of the valves by 

 small regular concentric wrinkles. 



This shell bears a general resemblance to Strophomena depressa. It 

 diflfers, however, remarkably from that species, in its resupinate character, 

 the convexity and concavity of the valves being in the opposite direction. 

 The area of the ventral valve is also wider than in that species, and the 

 cardinal margin is crenulated. 



Geological position and locality. In the shaly limestone of the Lower 

 Helderberg, Albany county. 



