116 CARBONIFEROUS PERIOD. 



and Iowa that I am quite unable to separate specifically from P. punctatus, 

 among which are the examples described by Hall (loc. cit.) under that 

 name, from the Keokuk limestone at Keokuk, Iowa, and Nauvoo, Illinois. 

 The collections contain comparatively few specimens of this species, a fact 

 probably due less to its scarcity or absence at other localities than to the 

 well-known fragility of the shell, which has doubtless prevented its perfect 

 preservation, such as we find in the case of many other shells associated 

 with it. 



Position and locality. — Strata of the Carboniferous period ; at and near 

 the top of Grass Mountain, Ely range, thirty-five miles north of Pioche, 

 Nevada. 



Productus Nebrascensis Owen. 

 Plate VIII, fig. 3 a, h, c, and (?. 



Productus Nehrascensis Owen, 1852, Geol. Report Iowa, Wisconsin, and Minnesota, 584. 



Productus Rogersi Norwood and Pratten, 1854, Jour. Acad. Nat. Sci. Phila., n. s., iii, 9. 



Productus Rogersi Hall, 1856, Pacific Railroad Report, iii, 104. 



Productus asper McChesney, 1860, Descr. New Paleozoic Fossils, 34. 



Strophalosia horrescens Geiuitz, 1866, Carbonformat. uud Dyas in Nebraska, 81 (not 



Murchison, de Vernenil, and Keyserling, 1845). 

 Productus Nebrascensis Meek, 1872, U. S. Geol. Sur^'. Nebraska, 165. 



Shell of about average size for a species of this genus ; outline, in front 

 of the cardinal border, suboval or sub-hemispherical ; length usually less than 

 the breadth ; hinge generally less in length than the greatest breadth of the 

 shell, and seldom equaling it; antero-lateral margins strongly, and front 

 margin broadly, rounded, the latter sometimes a little emarginate ; postero- 

 lateral margins somewhat straightened upon, and in front of, the ears, 

 meeting the cardinal border at a somewhat obtuse angle ; ears small, seldom 

 prominent. 



Ventral valve somewhat regularly convex from front to rear ; greatest 

 convexity behind the middle ; umbo prominent, projecting behind the hinge- 

 line ; beak prominent, incurved a little over the cardinal margin ; a mesial 

 flattening, amounting sometimes, but rarely, to a distinct sinus, extending 

 fi'om the umbo to the front margin. Dorsal valve flattened in the visceral 

 region, the antero-lateral and front portions curving abruptly upward ; beak 



