122 



[ Senate 



Atrypa impressa (n. s.). 



Pal. N.Y. Vol. iv. 



Shell longitudinally obovate or ov^al : dorsal valve the larger, 

 very gibbous, most elevated above the centre, often having a 

 ridge in the middle, along which is generally a faint sulcus ; 

 dorsal margin excavated on each side of the beak, for the recep- 

 tion of the dental laminse of the opposite valve ; beak distinctly 

 incurved : ventral valve depressed-convex, most prcmintnt in 

 the umboual region and along the middle ; beak closely in- 

 curved upon the opposite. Surface marked by numerous small 

 bifurcating radiating strire, which are crossed by distinct, 

 crowded, concentric, imbricating lamellose lines of growth. 



This shell resembles very closely the ■well-known A. reticularis, and 

 has heretofore been referred to that species. I have long been satisfied, 

 however, that it is a distinct species. It may always be distinguished from 

 the A. reticularis by the prominent, broad, flattened space down the 

 middle of the dorsal valve, which is bounded by obtusely angular margins, 

 and often depressed in the centre upon the exterior shell, and always in the 

 cast. Owing to this character, the front is usually truncate or emargiuate, 

 a feature not well represented in the accompanying illustrations. The ven- 

 tral valve is convex along the middle quite to the front, instead of being 

 depressed or sinuate as in A. reticularis. In a few instances, the front of 

 the ventral valve, though not sinuate, is slightly produced into a short 

 rounded projection. There are also internal differences between this shell 

 and A. reHcularis. 



Geological positioji and locality. Schoharie grit. 



Atuypa impressa. 



