218 BULLETIN UNITED STATES GEOLOGICAL SUEVEY. [Vol.V. 



beaks to tlie front; front ronnded witli a regular curve to the base; 

 basal margin broadly convex ; posterior margin short, abruptly trun- 

 cated downward and backward, forming a distinct, slightly obtuse angle 

 with the dorsal margin; the latter margin broadly concave from the 

 beaks to posterior margin ; beaks small, placed near the anterior end of 

 the shell, slightly elevated and turned a little forward. Surface marked 

 by concentric lines and wrinkles of growth. Extending from behind 

 the beaks to the narrow posterior end of the shell, there are upon each 

 valve two narrow obscure folds, which increase in width posteriorly, 

 with a narrow shallow sinus betw^een them. The lower one of these 

 folds is the broader, and ends at the postero-basal angle; and the upper, 

 which is involved with the infiexed dorsal border, ends at the postero- 

 dorsal angle. 

 Length, 29 millimeters; height, from base to beaks, 12 millimeters; 



. thiclkuess, both valves together, 7^ millimeters. 



This shell departs widely in aspect and form from the typical forms of 

 Allorisma, and it perhaps does not strictly belong to that genus, although 

 it corresponds in all essential respects with the generic diagnosis given 

 by its author. It is evidently congeneric with the sjjecies described by 

 Meek in the Final Eeport on the Geology of i^ebraska, p. 217, pi. x, fig. 15, 

 under the name of Allorisma {SedgicicMa) rejiexa. That species is from 

 the Upper Coal-measures at Nebraska City, and has also been found in 

 the Lower Coal-measures of Illinois, as stated by Meek. Compared with 

 that shell, ours is much more elongated, and the beaks are placed pro- 

 ]3ortionally nearer the front. 



Position and locality. — Carboniferous strata, associated with the two 

 preceding and three following species at Wild Band Pockets, Arizona, 

 15 miles south of Pipe Spring. Collected by Mr. G. K. Gilbert, in whose 



ihonor the specific name is given. 



I 



' ^ ,:-.:ffmmm^^...,^^ GASTEEOPODA. 



i . Genus Bellerophon Montfort. 



\> 



Bellerophon subpapillosus White. 



Among the other fossils herein noticed which Mr. Gilbert obtained 

 from Wild Baud Pockets, Arizona, are some imperfect specimens of a 

 Bellerophon which is closely related to B. urii Fleming and B. carhona- 

 rius Cox. In Powell's Eeport on the Geology of the Uinta Mountains, 

 p. 92, 1 gave a brief description of this form under the name B. carhona- 

 rius var. subpapillosus, the specimens having been obtained from near 

 the top of the Carboniferous series at different localities in Northwestern 

 Colorado. In view of the constancy of the characteristics by which it 

 differs from -Z>. carhonarius, I now prefer to treat it as a distinct species, 

 although I have no doubt of its genetic relation to that species. It is 

 about double the size of B. carhonarius. This characteristic w^ould be 

 of little value in a single instance, but it is constant in all cases and in 



