SDBCAllBONIFEROLTS PEllIOD. 87 



* 



strongly incurved and projecting a little over the hinge-line; mesial fold 

 distinctly defined from the front margin to the apex of the beak, where it is 

 minute, but widens a little more rapidly near the front than elsewhere ; it is 

 not much elevated above the adjacent parts of the valve, but is distinctly 

 separated from them by an interspace on each side of it, which is a little 

 deeper and wider than those which separate the plications from each other. 



Ventral valve regularly arching from rear to front; sides somewhat 

 regularly convex, but like the other valve it is a little depressed at the 

 lateral extremities; beak prominent, strongly incurved, abruptly narrowing 

 to a point ; mesial sinus distinctly defined from the front border to the apex 

 of the beak, nowhere very deep, increasing uniformly in width toAvard the 

 front; area moderately naiTow, striated transversely; foramen triangular, 

 nearly equilateral. 



Surface marked by from thirty-four to forty -two small, rovmded, dis- 

 tinct plications, nearly or quite all of which are simple; the two plications 

 that form the lateral boundaries of the mesial siniis a little larger than any 

 of the others, gradually but slightly diminishing in size from the sinus to 

 the lateral extremities, where they become obsolete. From four to six pli- 

 cations are foxind upon the mesial sinus and fold respectively, all of which 

 are similar to those iipon the sides of the shell, except that the former 

 diminish more rapidly in width toward the beaks. Concentric lines and 

 undulations exist upon both valves. 



Breadth at the hinge-line, aljout thirty-eight millimeters; length from 

 beak to front, twenty-two millimeters; thickness, sixteen millimeters. 



Professor Winchell's type-specimens were obtained from the Subcar- 

 boniferous strata at Cuyahoga Falls, Ohio, and seem, from the description, 

 to have consisted of venti'al valves only. His carefully-di-awn description 

 of that portion of the shell agi-ees so very closely with the corresponding 

 part of oiirs that I should entertain little or no doubt of the specific identity 

 of the two if it were not that he speaks of his shell as being very nearly 

 like S. cuspidatus Hall (not Martin), which I understand to differ consider- 

 ably from ours. 



Position and locality. — Subcarboniferous strata; Mountain Spring, old 

 IMormon road, Nevada. 



