INVERTEBRATE TALyEONTOLOGY. 591 



to two-thirds as wide as the outer whorl, and rather deep; aperture trans- 

 versely oval ; surface obscurely marked by very fine lines of growth. 



Greater diameter, 0.12 inch; height, 0.04 inch. 



When somewhat distorted by pressure, or partly embedded in the 

 matrix, this little shell, in consequence of its depressed spire, presents almost 

 exactly the appearance of a Planorbis, to which genus we at first referred it. 

 When entirely detached from the matrix, however, it is found to possess the 

 characters of a true Valvata. It seems to be nearly related to some of the 

 depressed forms of V. tricarinata, Say, particularly the variety simplex, 

 which like our shell is not carinated. Yet it is uniformly smaller, more 

 depressed, and appears to be always entirely destitute of the slightest traces 

 of carina'. 



Locality and position. — Three miles below Fort Union, Dakota; from a 

 ferruginous band of impure lignite, in the Fort Union group of the Fresh- 

 and Brackish-water Lignite series ; probably Lower Eocene. 



Valvata parvnla, M & H 



Valvata parvula, Meek and Haydcu (1856), Proceed. Acad. Nat. Sci. Philad., VIII, 123. 



Shell very small, depressed or subdiscoidal ; spire rising little above 

 the body-whorl ; volutions about three, rounded, and having, near the middle 

 of the upper and lower sides, a distinct linear carina; surface marked by 

 strong, regular lines of growtn ; suture distinct; umbilicus wide and deep, 

 showing all the volutions to the apex; aperture rounded or transversely oval. 



Greatest breadth, 0.10 inch; height, 0.05 inch. 



The only specimen of this shell found was unfortunately lost soon after 

 it was described, so that I have been unable to give any figures of it. My 

 present impression, however, is, that it is possibly only an angular variety of 

 the last, though it may be found to be distinct. 



Locality and position. — Same as last. 



Valvata! IVIoiitanaensis, Meek. 



Shell attaining a rather large size, depressed-sub- Fi "- 81 - Fi s- 82. Fig. 83. 

 globose; volutions three, rather rapidly increasing in (^ ^^ £^5) 

 size, particularly in breadth; compressed-convex, both _ , Jt ,, J 



1 •> x Valvata Montanaensu. 



above and below ; periphery in young and medium- Three views, about one 



j ill l and a half diameters. 



sized specimens angular, but becoming narrowly 



rounded in the adult ; suture deep ; umbilicus small ; aperture about as wide 



