ATRYPA OP THE UPPER HELDERBERG GROUP, &c. 
Atrypa psemloniarginalis. 
PLATE LIII. 
Atrypa pseudomarginalis: Hall, Thirteenth Report on the State Cabinet, p. 84. 1860. 
Shell trilobate, wider than long; surface plicated. 
Ventral valve depressed-convex, gently curving to the sides; mesial sinus 
deeply depressed in the lower part, and extending for more than two- 
thirds the length of the shell; beak extending beyond that of the dorsal 
valve and moderately incurved. 
Dorsal valve regularly convex above, curving somewhat abruptly to the 
sides; the mesial fold becoming conspicuous at about one-third the length 
from the beak and rising into a prominent rounded lobe, which occupies 
more than one-third the width of the shell in front. 
Surface marked by numerous rounded bifurcating plications. 
This species resembles Atrypa marginalis of Dalman, but is larger and more 
robust than the specimens of that species which I have seen : the beak is less 
attenuate, the mesial fold and sinus broader and not extending to the beak, and 
the striae coarser and not so much recurved. From the Bohemian specimens under 
the same name, it differs in the greater elevation and rounded form of the mesial 
lobe, and the less angular plications. It has proved extremely rare in the rocks 
of New York. 
Geological formation and locality. In the Upper Helderberg limestone at Scho¬ 
harie, New York. 
