136 UNITED STATES GEOLOGICAL SURVEY OF TUE TERRITORIES. 



within, deeply rounded in front, and contracted behind; posterior side nar- 

 rower than the other, and subtruncate at the extremity; beaks small, 

 moderately prominent, pointed, and very nearly central; surface polished, 

 and marked by fine, rather regular, concentric lines ; ligament depression 

 lanceolate ; lunule smaller, but nearly of the same form, and deeply excavated ; 

 while on each side of it there is usually a more or less defined sulcus 

 extending forward to the upper part of the anterior margin. 



This shell may possibly belong to a distinct species from the last, 

 instead of being a mere variety of the same ; but both vary to such an extent 

 that I am now inclined to think the form under consideration more probably 

 only a variety of the occidentaUs. Its chief differences are its smaller size, 

 thinner shell, and proportionally rather shorter and more rounded outline; 

 its anterior ventral margin being more deeply rounded. It also has a rather 

 more finely striated and smoother surface, and generally differs in having a 

 shallow sulcus extending from the beaks, on each side of the lunule, to the 

 upper part of the anterior margin. 



In general form, as well as in size, and the presence of the anterior 

 sulcus, it approaches more near the first of the forms described; but it differs 

 materially in its surface-characters, being destitute of any traces of the con- 

 centric undulations seen on that shell, which also presents a rougher appear- 

 ance, as if it had been covered by a thin epidermis. Still, it may be possible 

 that all three of the types here described are merely varieties, or represent 

 different ages of the same species. Indeed, I confess that I am not alto- 

 gether satisfied with the conclusion that they belong to more than one 

 species; but, having already separated them, it is perhaps better to continue 

 this arrangement than to unite the whole under one name, doubtfully. 



Locality and position. — -Yellowstone River, 150 miles from its mouth ; 

 in beds containing a blending of the fossils of the Fort Pierre and Fox Hills 

 groups of the Upper Missouri Cretaceous series. 



Genus SPHJERIOLA, Stoliczka. 



Synon. — Spkcera (up.), d'Archiac, and some others. 



Sphceriola, Stoliczka (1871), Palseont. Indica, III, 217. 



Etym. — Sphcera, a sphere (diniin.). 

 Typo- — Sphaira Madridi, d'Archiac. 



Shell globose or subglobose, nearly or quite equilateral, with free 

 margins closed and smooth within; surface with concentric stria? or sulci; 



