578 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



dations connecting these different forms. In some specimens, the upper part 

 of the upper whorls is slightly compressed, while in others they are all regu- 

 larly rounded. Sometimes all the volutions are faintly compressed on their 

 upper outer slopes, so as to give the shell a very slightly subtrochiform 

 aspect. Generally, the minute revolving striae and small punctures are 

 entirely obsolete, and they are never visible without the aid of a magnifier. 



This species is somewhat similar to V. semicarinatus, Brard, as figured 

 by Deshayes (Coq. Foss., II, pi. 5), from the Paris basin ; but its aper- 

 ture is more ovate in form, and its umbilical perforation smaller. It seems 

 to bear about the same relations to the following species that V. semi- 

 carinatus does to V. Desnoyesii of Deshayes. 



Among recent American species, it may be compared (in form) with the 

 V. subpurpura, Say, from which, however, it will be readily distinguished by 

 its more depressed spire, and more ventricose and more rounded whorls. It 

 is also a thicker shell, and approaches more nearly some East Indian forms. 



Locality and position. — Three miles below Fort Clarke, Dakota; in the 

 Fort Union Lignite group (probably Eocene). 



Viviparns r e t u s u s , M. & H. 



Plate 44, fig*. 5, a, b, c, <l, e,f. 



Pahidina return, Meek and Hayden (1856), Proceed. Acad. Nat. Sci. Philad., VIII, 122. 



Vivipara retusa, Meek and Hayden (1860), ib., XII, 185. 



Piriparus retusns, Meek and Hayden (1863), in Gill's Syst. Arrang. Fam. Yiviparidm, 6. 



Shell of medium size, obliquely subovate in outline; spire rather 

 depressed-conical, obtuse at the summit ; volutions five to five and a half, 

 more or less convex; last one generally more than equaling the height of the 

 spire, obliquely flattened or sometimes a little concave near the suture, prom- 

 inently rounded or obscurely subangular a little below the middle; suture 

 well defined; surface marked by distinct oblique lines of growth, with some- 

 times on the upper whorls very faint traces of extremely minute revolving 

 striae; aperture broad-ovate, somewhat angular above, and rounded below; 

 inner lip a little thickened, and closely flattened upon the deeply arcuate, 

 imperforate columella; outer lip very oblique in outline. 



Length, 1.13 inches; breadth, 0.90 inch; apical angle convex, diver- 

 gence varying with age from 90° to 70°. 



This species resembles the last somewhat, but will be at once distin- 

 guished by always having its upper whorls more depressed, so as to makethe 



