70 TRENTON PERIOD. 



area sharp ; anterior and lateral margins of the valve thickened and 

 roughened by the vascular markings, which are much less distinct within 

 the tliickened border than they are upon it. 



Surface of both valves marked by fine, close-set, radiating striae, often 

 of somewhat unequal size, the smaller ones being those that are implanted 

 at various distances from the beak between those that are continuous from 

 it to the margins ; under a lens, the radiating striae are seen to be finely 

 crenulated by the crossing of numerous delicate concentric striae. 



Compared with authentic specimens of S. filitexta, and also with those 

 of associated species to which it is closely related, our specimens coiTespond 

 most nearly with those of the species to which they are here refeiTed, 

 although they present some slight differences. 



Length, twenty-two millimeters ; breadth, thirty-four millimeters. 



Position and locality. — Strata of the Trenton period, Cincinnati epoch; 

 Silver City, New Mexico. 



Genus LEPT.ZENA Dalraan, 1828. 



Leptaena sericea Sowerby (?). 

 Plate IV, fig. 7. 



Among the fossils collected at Silver City, New Mexico, from strata 

 that are unmistakably referable to the Cincinnati epoch, are a few imperfect 

 specimens of Leptcena that I refer with some doubt to the species generally 

 recognized in America as L. sericea Sowerby. The specimens are too 

 impei-fect to base a specific description upon, but enough is shown to indicate 

 that if they are not specifically identical with L. sericea, they belong to a 

 closely-related and representative species. 



Genus OETHIS Dalmau, 1828. 



Orthis occidentalis Hall. 



Plato IV, fig. 1 1 a and b. 



Orthis occidentalis Hall, 1847, Paleontology of New York, i, 127. 



Orthis simiata Hall, ib., 128. 



Orthis subjitgata Hall, ib., 129. 



Orthis occidentalis Meek, 1873, Paleontology of Ohio, i, 96. 



Shell moderately large, suboval or subquadi'ate in outline, the trans- 



