INVERTEBRATE PALEONTOLOGY. 575 



to this group, though I have seen in the Smithsonian collection from the Isle 

 of Wigit a Tertiary species apparently nearly related to it. 



In regard to the geological range of this type, we at present only know 

 certainly, that it occurs in the Fort Union group of the Upper Missouri 

 Lignite series. 



M i c r o p y r g u s minutulus, M. & H. 



Plate 43, figs. 18, a, b. 



Melania minutula, Meek and Hayden (1856), Proceed. Acad. Nat. Sei. Pbilad., VIII, 123. 

 Micropyrffw minutulus, Meek (1866), iii Conrad's Smithsonian Check-List N. Am. Invertebrate Eocene 

 Fossils, 12. 



Shell very small, subcylindrical ; spire about twice the length of the 

 aperture, with its obtuse apex smooth ; volutions seven and a half to eight, 

 convex, and increasing gradually in size from the apex, each compressed or a 

 little flattened above, with an oblique outward slope for about two-thirds of 

 its breadth down from the suture, thence rounding inward rather abruptly to 

 the suture below, so as to form a subangular revolving prominence, which is 

 continued around the middle of the body-whorl ; surface smooth, or only 

 showing very obscure traces of minute lines of growth ; aperture rhombic- 

 ovate, angular above, and subangular and a little produced at the slightly 

 effuse base. 



Length, 0.18 inch; breadth, 0.07 inch; apical angle regular, diverg- 

 ence 20°. 



This little shell differs materially from either of the foregoing, not only 

 in its smaller or minute .size, but also in its elongate slender form, and the 

 peculiar manner in which its flattened whorls project just above the suture. 

 In this latter character, as well as in its slender elongated form, it resembles 

 an Eocene Tertiary shell described by Deshayes under the name Auricula 

 spina (Coq. Foss., II, pi. 8, figs. 10-11); but it is considerably smaller, 

 with not near so many whorls, while it shows no traces of a tooth on the 

 columella. Its resemblance to Deshayes's species is probably a mere simi- 

 larity of form only, since the two shells seem to be generically, if not more 

 widely, distinct. 



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

 Fort Union Fresh- and Brackish-water Lignite group; probably Eocene. 



