120 CARBONIFEROUS PERIOD. 



Productus muricatus Norwood aud Priitten. 



Plate VIII, fig. 4 a, b, and c. 



Prodtwtus murwatus Norwood aud Pratten, 1854, Jour. Acad. Nat. Sci. Phila., n. s., 

 iii, 14. 



Among the collections made by Prof. J. J. Stevenson from the "Mid- 

 dle Division, Carboniferous, Rock Creek, Lake County, Colorado", are some 

 imperfect but unmistakable specimens of P. muricatus Norwood and Prat- 

 ten. This is quite a well-known form; but Mr. Meek (U. S. Geological 

 Sm-vey of Nebraska, page 161) refers it with doubt to P. longispinus. 

 Having found, in a long familiarity with all these shells, that, unlike most 

 of the admitted varieties of P. longispmiis, this species retains its specific 

 characteristics with great constancy over wide areas, I am disposed to 

 regard it as a distinct species. 



So far as at present known, the geological range of the species is 

 tlu'ough the whole of the Carboniferous or Coal-Measure series. In Iowa, 

 I found it almost characteristic of the Middle Coal-Measures, in the lime- 

 stones and calcareous shales of Avhich it was often found abundant. 



Productus Mexicanus Shuraard (?). 

 Plate VIII, fig. 6 a, h, and c. 

 Productus Mexicanus Shumard, 1858, Traus. Acad. Sci. St. Louis, i, 291. 



Shell small, usually wider than long, emarginate in front, and flattened 

 a little at the sides, giving it a somewhat four-sided outline when viewed 

 vertically ; cardinal border equal to, or greater than, the greatest breadth 

 of the body of the shell ; ears more or less prominent, distinctly defined and 

 convex upon the ventral side. Ventral valve strongly and somewhat regularly 

 arched, the. curvature being greatest posteriorly ; beak somewhat prominent, 

 and slightly incurved over the cardinal border ; mesial sinus obsolete or 

 indistinctly defined, and disappearing at the visceral region. 



Surface marked by from sixteen to twenty rounded, radiating costse, 

 with interspaces of similar width, all of which are most distinct upon the front 

 part of the shell ; the visceral region marked by concentric wrinkles, and 

 the whole sm-face by fine concentric lines of growth; strong erect spines 

 are scattered over the surface of the ventral valve, borne upon the costa?. 

 Dorsal A^alve unknown. 



