340 GEOLOGY OF THE BLACK HILLS. 



ber, so that the comparison is not a favorable one ; still there is no doubt 

 of the correct identification, as the differences are no more than might 

 result from local causes. 



Formation and locality.— In friable sandstone of the Potsdam forma- 

 tion, at the head of Redwater Canon, Black Kills. 



OBOLELLA NANA. 



Plate 2, figs. 14-17. 



OboIeUa nana M. & H., Proc. Acad. Nat. Sci. Phil., Oct. 1861, p. 435. 

 OholeUa nana M. & H., Pal. Upp. Missouri, \}. 4, PI. 1, fig. 1. 



" Shell very small, subcircular or transversely suboval ; moderately 

 convex; rather thick; front broadly rounded; sides more naiTOwly rounded. 

 Beak of dorsal valve short and obtuse. Ventral valve proportionally a 

 little longer than the other, or about as wide as long, and having a slightly 

 more prominent beak ; without a distinct mesial ridge within ; scar of 

 anterior adductor muscle ? located behind the middle ; impressions of 

 sliding muscles narrow ; cardinal area rather narrow and flattened ; groove 

 for the passage of the peduncle shallow. Surface marked by a few con- 

 centric grooves or impressed strife ; exfoliated specimens showing small, 

 regularly disposed radiating striae." 



The above is the description of the species as given by its authors in 

 the Paleontology of the Upper Missouri, and agrees well with the many 

 specimens before us from the same region of country. But we notice 

 among them considerable diversity of form ; different individuals varying 

 from transversely oval to elongate tiiangular-ovate, as will be seen by refer- 

 ence to the figures on Plate II. The specimens vary very much also in 

 the degree of convexity, many of them being flattened-discoid, while others 

 are highly ventricose or become quite gibbous in the vicinity of the umbones. 

 It is possible there may be more than one species among the specimens 

 referred by us to this one ; but as on many blocks, bearing an aggregate 

 of several hundred individuals, the general similarity is so great, wliile the 

 individual characteristics are so variable, we should not hesitate for a 

 moment to consider them all as of the same species. One feature is quite 

 noticeable, however, among them, that the transversely broad-oval speci- 



