24 Natural History Bulletin. 



from nearly flat to strongly concave, and the angle that the 

 areas of the two valves make with each other at the hinge 

 line is also quite inconstant. 



In all cases the convexity of the two valves is approximately 

 equal, the mesial fold and sinus are well defined to the beak, 

 and the valves are quite regularly convex, both longitudinally 

 and transversely. In average specimens the dorsal valve is 

 sub-elliptical, the transverse being to the vertical diameter 

 about in the ratio of twenty to thirteen, with the cardinal 

 angles measuring about ninety degrees. The mesial fold is 

 low, flattened or evenly rounded from side to side, distinctl}^ 

 set off from the lateral areas by furrows somewhat deeper 

 than usual, in many instances not rising above the general 

 convexity of the valve, though generally more or less elevated. 

 The mesial sinus of the ventral valve is broad and shallow, 

 rapidly widening to the front margin and evenly concave from 

 side to side. A more or less perfectly defined sub-angular 

 ridge marks the boundary between th6 sinus and the lateral 

 areas. Two well defined furrows in the sinus, situated re- 

 spectively about a fourth of the width of the sinus from each 

 margin, is an interesting feature apparently affecting all the 

 individuals from certain localities. From other localities the 

 specimens of this species show no trace whatever of furrows 

 in the sinus, while from still other localities a part of the in- 

 dividuals may exhibit them in varying degrees of perfection. 

 The furrows seem to mark the boundaries of two rather 

 broad rudimentary plications, one on each side of the sinus. 

 There are no corresponding indications of plications on the 

 mesial fold. 



The surface is marked by from eighteen to twenty-two 

 plications on each side of the mesial fold and sinus. The first 

 twelve or fourteen of these plications are distinct, the remain- 

 der being small, rudimentary, and more or less obscure. In 

 typical specimens the plications are low and flat, and separated 

 from each other by narrow furrows, but this, like nearly all 

 he other characters, is exceedingly variable. The plications 



