556 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



The figured specimen is the only one in the collection certainly known 

 to belong to this species. It has been distorted somewhat by compression, 

 which gives slightly greater breadth to the lower volutions than is natural. 

 It may be compared with Achatina Rillyensis, Boissy, from the Inferior 

 Lacustrine beds of the Paris basin, which species seems to be a true Columna. 

 Our shell, however, when not distorted, has its lower volutions merely flat- 

 tened or slightly convex, and not concave, and the suture between them 

 much more oblique. Judging from the form of its last volution, its aperture 

 would also seem to have had a quite different outline. 



I have not had a very satisfactory view of the aperture of this or the 

 following forms, and consequently only place them provisionally in the genus 

 Columna. In working out one of the specimens, however, 1 had a partial 

 view of its aperture before it broke to pieces ; and its columella was seen to 

 be twisted as in that genus, and apparently provided with a single prominent, 

 oblique ridge, as in the same. Before seeing this, I had once doubtfully 

 placed these shells in the genus Clausilia. This character of the existing 

 columella, however, and the fact that the latter genus is not known among 

 the mollusks of the continent (proper) of North America, are the reasons 

 that lead me to retain them here under the genus Columna. It is true 

 that the typical forms of the latter genus are equally foreign to this country; 

 but the section R/iodea, of the same, occurs in California, while our shells 

 appear to present intermediate characters between that group and the typical 

 forms of the genus Columna. 



Locality and position. — Fort Union group, near Fort Union, Dakota, 

 on the Upper Missouri ; probably Lower Eocene. 



Co I ii in 11 a v e v in i c n 1 a , M. & H. 



Plate 44, figs. 12, a, b. 



Bulimun vermiculus, Meek and Hayden (1856), Proceed. Acad. Nat. Sci. Pbilad., VIII, 118. 

 Columna ? vermicula, Meek and Hayden (1860), ib., XII, 431. 



Shell small, sinistral, very slender, or elongate-subcylindrical ; volutions 

 about thirteen, narrow, closely coiled, and increasing very gradually in size, 

 slightly convex near the summit of the spire, but nearly flat farther down ; 

 suture merely linear between the lower whorls, but becoming more distinct 

 nearer the apex ; surface marked by tine obscure lines of growth, crossing 

 the volutions at right angles to the suture. Aperture unknown. 



