CARBONIFEROUS PERIOD. 117 



and auricular regions depressed so as to produce a slightly-raised, rounded, 

 diverging fold between them, respectively, s.t each side ; mesial fold seldom 

 distinct, and perceptible only at the front. 



Sui-face of both valves covered with numerous spines of different sizes, 

 but all very small ; those of the ventral valve are borne upon more or less 

 distinctly defined concentric folds, and may be divided into two sets, or 

 kinds, one consisting of the stronger and more erect spines, and the other 

 of small, short ones, which latter are closely appressed against the surface ; 

 both kinds are more or less connected by means of numerous raised, radiat- 

 ing lines, which are apparent upon the concentric folds, but scarcely so 

 upon the surface of the interspaces. 



Length, thirty-three millimeters ; breadth, thirty-five millimeters. 



As usually obtained from a limestone matrix, the spines and more or 

 less of the surface of the shell remain with the matrix. Such specimens 

 present an appearance sodifferent from thatof those which have been perfectly 

 preserved in a soft matrix that they have been referred to different species. 

 Among references of this kind is that made by Professor Geinitz (loc. cit.) 

 of this species to Strophalosia liorrescens. Concerning this I can only say 

 that I have collected and examined hundreds of examples of this species 

 from his typical locality, and from numerous other localities in Nebraska, 

 Iowa, Illinois, and Missouri, many of them having the interior and all other 

 parts well preserved and shown, and not one of them was found to possess 

 any of the generic characters peculiar to Strophalosia. That, as suggested 

 by Dr. Geinitz, the genus Strophalosia may have been developed from Fro- 

 ductiis, and also that in some cases, while it was effecting such a generic 

 transition, the specific characters may have remained comparatively 

 unchanged, I am willing to believe ; but that question need not be now 

 discussed. That Productus Nehrascensis, however, affords any evidence of 

 such a change, I am by no means prepared to admit. 



Position and locality. — Strata of the Carboniferous period : — Camp 

 Apache and Carrizo Creek, Maricopa County, Arizona : Rubyville, Schell 

 Creek range ; and top of Grass Mountain, Ely range, Nevada : Meadow 

 Creek, south of Fillmore, Utah ; and other localities. 



