No. 109.] 143 



fore in doubt as to the correctness of the original reference of this shell. 



This species differs much in the convexity of the ventral valve ; being 

 sometimes extx-emely gibbous, and in other individuals very moderately 

 convex. The area is subject to some slight variations in width, perhaps 

 owing to compression. The general character of the surface of the convex 

 valve is much like that of good specimens of Strophomena alternata of the 

 Trenton limestone, but it does not appear to be subject to such extreme 

 variations in respect to the striae. It resembles very closely, in its surface 

 markings, the S. Itxtilis; but is always a smaller shell, has a more ex- 

 tended hinge-line, and is never so flat as that species. 



Geological position and locality. In the shales of the Hamilton group: 

 Shores of Seneca and Cayuga lakes, Canandaigua lake, Moscow, Geneseo, 

 etc. 



Strophomena ( Strophodonta) fragilis ( n. s.). 



Pal. N.Y. Vol. iv. 

 Shell semioval ; hinge-line often extended beyond the width of 

 the shell ; margins a little contracted below the hinge extremi- 

 ties : dorsal valve scarcely concave : ventral valve very slight- 

 ly convex, or nearly flat ; area very narrow, linear, almost 

 entirely formed by the ventral valve, striated vertically, and 

 crenulate on the inner edges throughout the entire length ; fo- 

 ramen none. Surface covered by minute closely arranged ra- 

 diating striae; and these are crossed by fine concentric lines, 

 which become very prominent on the radiating strise of the 

 ventral valve, giving a minutely nodulose or granulate character 

 to the entire surface when well preserved. The radiating strise 

 of the dorsal valve are more evenly rounded, and increase 

 mainly by bifurcation (while those of the opposite valve increase 

 by interstitial additions), and are crossed by even, raised, con- 

 centric lines which produce no granulations upon the surface. 

 The dorsal valve is frequently marked by faint concentric un- 

 dulating wrinkles ; while the ventral valve often presents some 

 short oblique folds just below the hinge-line, and very rarely a 

 few short interrupted scarcely perceptible undulations upon the 

 general surface. 



Vascular impressions of the ventral valve double, foliate, 

 without central dividing ridge, margined by a slightly elevated 

 ridge on each side for a short distance below the hinge-line. 



