PEIMOBDIAL FOSSILS. 339 



The specimens present in the collection are in a coarse friable sand- 

 stone and very imperfect, or in a harder band packed so closely together 

 that it is impossible to isolate a perfect valve from those surrounding, so 

 that the features are much obscured. The substance of the shell is the 

 same as those used in the original description of the species, loo. cit , and 

 on exfoliation presents the same peculiar radiating lines there indicated. 

 The imperfect material has not afforded any further knowledge of the 

 interior structure or muscular makings than there given. 



Formation and locality. — In dark brown sandstone layers of the Pots- 

 clam formation, on Castle Creek, Black Hills, Dakota. 



Genus OBOLELLA Billings 

 OBOLELLA POLITA. 



Plate 2, figs. 12, 13. 



Obolns apollinus t Owen (not Eichwald), Geol. Surv. Wiscon., Iowa, and Minn., p. 501, 



etc., Tab. 1 B, figs. 9, 11, 15, 20. 

 Idngula f polita Hall, Ann. Geol. Bept. Wisconsin, 1860, p. 24. 

 Obolella ? polita Hall, 16th Bept. State Cab., p. 103, PL vi, figs. 17-21. 



Shells small, short-ovate, being much the broadest below the middle 

 of the length and narrowing above, somewhat abruptly so to the small, rather 

 pointed beak of the ventral valve; basal line broadly but regularly rounded ; 

 surface convex, rather strongly rounded, most abruptly so in the upper 

 part and becoming prominent and highly rounded toward the beak on the 

 ventral valve, and more evenly rounded on the dorsal ; surface of the 

 shell smooth, or showing lamellose lines of growth on the weathered or 

 partially exfoliated specimens. Interior of the shell not observed in any 

 of the specimens present in the collection. 



The shells referred to this species among the collections from the 

 Black Hills differ very little from those from Wisconsin ; they are rather 

 more regularly ovate, not so wide below, nor so regularly narrowing 

 toward the beaks, as the generality of those from that locality; the surface 

 is slightly more lamellose and not so polished ; but the specimens are all 

 more or less weathered and less perfectly preserved, and are few in mun- 



