724 



NORTH AMERICAN INDEX FOSSILS. 



(lower) side ; umbilicus covered by callus. Generally covered 

 with shell fragments. 



Chipolan (Oligocenic) of Gulf States ; Chesapeakean (Miocenic) 

 of Atlantic coast. 



Fig. 1053. Xenophora conchyliophora, X %• (Md. Survey.) 



Family Valvatid^e Gray. 

 CXVIII. Valvata Muller. 

 Small umbilicated shells, of naticoid form with few regularly en- 

 larging whorls, forming a conical or discoidal spire ; aperture 

 circular, peristome continuous ; operculum horny, multispiral. 

 Habitat fresh water. Jurassic-Recent. 



399. V. scabrida M. arfd H. (Fig. 1054, a-c.) Jurassic. 

 Minute, rather loose-coiled, with obtuse conical spire and deeply 



impressed sutures ; aperture circular. 



Como (Atlantosaurus) beds of Black Hills, Morrison of 

 Colorado. 



400. V. nana Meek. (Fig. 1054, d } e.) Cretacic. 





FlG. 1054. a-c, Valvata scabrida; d, e, V. nana ; f, g, V. subumbilicata. (All 

 enlarged.) (U. S. G. S.) 



Minute, spire very low, approaching flatness, whorls in contact 

 but not impressed. 



Coalville (Coloradoan) of Utah. 

 401. V. subumbilicata M. and H. (Fig. 1054,/,^.) Cretacic. 



Larger than preceding (nearly three times as large), few-whorled, 

 spire low, umbilicus small. 



Laramie formation of the upper Missouri River region. 



