ATRYPA OF THE UPPER HELDERBERG GROUP, &c. 327 



Atrypa psendomarginalis. 



PLATE LIII. 



Mrypa pseudomarginalit : Hall, Thirteenth Report on the State Cabinet, p. 84. 1860. 



Shell trilebate, 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 Alrypa marginalis of Dalman, but is larger and more 

 robust thaa 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. 



Oeological formation and locality. In the Upper Helderberg limestone at Scho- 

 harie, New York. 



