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GENUS RHYNCHONELLA (Fischer). 



Rhynchonella acinus (Hall). 



Shell small, longitudinally ovate, subattenuate towards 

 the beak, and truncate in front, valves subequally con- 

 vex. Ventral valve subarcuate, flattened in the middle, 

 below which it is sinuate ; beak incurved. Dorsal valve 

 somewhat flattened in the middle, and sometimes a little 

 depressed in the upper part of the median line, two of 

 the plications becoming elevated towards the front, cor- 

 responding to an abruptly depressed sinus in the ventral 

 valve, in the bottom of which is a single plication. There 

 are three and rarely four plications on each side of the 

 mesial fold of the dorsal valve, and four on each side 

 of the sinus of the ventral valve. The concentric lines 

 of growth are usually but faintly marked. 



The specimens are from one fourth to three eighths of an 

 inch long, the length and breadth being usually about as 

 four to three, and the depth about equal to the width, giving 

 a subquadrate transverse section. 



This species differs from the R. bidentata, of Hisinger, in 

 being larger, more robust and ventricose, and proportion- 

 ally more elongate ; the plications are more rounded, and 

 the whole aspect less angular. It approaches in form the 

 R. bialveata of the lower Helderberg group ; but is more 

 robust, with the plications more rounded. 



Rhynchonella indianensis (Hall). 



Shell broadly ovate or subtriangular, length and width 

 nearly equal, the width sometimes exceeding the length, 

 cardinal slopes in the more gibbous specimens flattened. 

 Ventral valve with the beak pointed and incurved, de- 

 pressed, convex in the middle, and gradually becoming 

 depressed and sinuate in front ; two or three of the plica- 



