140 [Senate 



Strophomena ( Stropiiodonta) concava ( n. s.)- 



Shell large, concavo-convex, hemispherical ; hinge crenulated, 

 generally less than the greatest breadth of the shell : dorsal 

 valve regularly and profoundly concave : ventral valve strongly 

 convex; area broad linear; margins nearly parallel, strongly 

 striate vertically, a little arcuate; foramen none. Surface or- 

 namented by rather abruptly angular distant elevated radiating 

 stricB, which always increase by implantation, and are crossed 

 by fine regular closely arranged elevated concentric lines. Where 

 worn or partly exfoliate I, the shell is finely punctate. 



This species resembles somewhat S. demissa, with which it is associated ; 

 but differs in being more deeply concave, and in generally having the hinge 

 shorter in proportion to the breadth of the shell : the striae are also pro- 

 portionally smaller and much more distant, especially near the beaks. 

 When well preserved, the stria; are rbruptly prominent upon the body of 

 the shell, and sharply crenulated by the concentric lines, which are less 

 conspicuous in the spaces between the striae. 



The shell varies much in its convexity, and is often somewhat irregular- 

 ly ridged or plicated in the direction of the strias. 



Geological posit io7i arid locality. Seneca lake, Moscow, York, and 

 many other localities in Western New- York. 



STROinOME.VA CONCAVA. 



