oo 



CAEBONIFEKOUS PERIOD. 13 



front by the greater or less curving-oiitward of the sides ; sides of the valve 

 sloping' almost directly from the mesial fold to the lateral borders ; antero- 

 posterior convexity of the mesial fold very slig-ht from front to middle, but 

 increasing from the middle to the beak ; beak small, projecting slightly over 

 the cardinal border. 



Ventral valve strongly arching from beak to front, the beak being 

 prominent, pointed, and curved over the area ; area concave, of moderate 

 width, and not narrowing to a sharp angle at the hinge-extremities; foramen 

 almost equilaterally triangular, partially closed by a pseudo-deltidium, which 

 is often removed by weathering ; mesial sinus well defined from front to 

 beak, and in all respects answering to the mesial fold of the other valve. 



Surface marked by numerous distinct, rounded strise of unequal size, 

 which increase gradually in size toward the front ; strise increasing in 

 number by the division near the beak of the few that are continuous to its 

 .point ; they are thus generally gathered into more or less distinct fascicles 

 of three or more striae in each, the middle strise of the fascicle being the 

 most prominent, and also the one that reaches the point of the beak ; the 

 mesial fold and sinus usually have striae of the same character and arrange- 

 ment as the sides of the shell have, but in some rare cases they are obsolete 

 upon the sides of the fold and sinus respectively. Besides the radiating 

 striai the usual concentric lines and occasional coarser marks of growth 

 exist. 



Length of a good-sized example, thirty-two millimeters ; breadth, at 

 the hinge-line, forty-three millimeters ; height, twenty-two millimeters. 



This is one of the most common species in American strata of the Car- 

 boniferous period, and may generally be identified without hesitation, but 

 there are some varieties of it that give more trouble in satisfactory identifica- 

 tion. One variety has been described by Professor Swallow as var. 

 Kansasensis, which is more than usually transverse, and the striae nearly 

 unifomi in size, and not fasciculated, as they are in typical examples, 

 approaching in these respects S. striatus Martin, sp. 



Position and locality. — Strata of the Carboniferous period : — Fossil HiU, 

 White Pine County ; Ely range ; Old Potosi Mine, Lincoln County, and 

 Egan range, thirty-five miles south of Egan Pass, Nevada ; Camp Apache, 



