No. 109.] 



115 



This species likewise bears considerable resemblance to Orihis (LepfcBna) 

 comigaia of Portlock, but is a larger shell as it usually occurs in our 

 mcks. It is not ^trofhomena cnrmgata., Conuad ( Jour. Acad. Nat. Sci. 

 Phil. 1842; Pal. N.Y. Vol. ii, 1852, pa. 59, pi. 21, f, 2). 



Geological posilio)i and locality. In the limestone of the Upper IlclJcr- 

 berg group, Albany and Seneca counties. 



' 3 



I 



Strophomena patersoxi. 



Strophomena concava (n. s.). 



Pal. N.Y. Vol. iv. 



Shell large, triangnlar-hemisplierical ; hinge-line equalling the 



greatest width of the shell : dorsal valve unknown : ventral 



valve profoundly concave, extremely gibbous in the umbonal 



region, from which a prominent rounded mesial lobe extends 



quite to the narrow rounded front; beak strongly incurved. 



Surface marked by numerous distinct, irregular, bifurcating, 



subangular strise. 



The most marked characters of this species are the deep concavity, 

 strongly incurved beak, and triangular form of its ventral valve ; which 

 presents also the somewhat unusual character of a prominent ridge down 

 the mi Idle, with indications of smaller ones on each side. 



Geological position and locality. Limestone of the age of the Upper 

 Helderberg, Western New- York. 



