INVERTEBRATE PALAEONTOLOGY. 573 



shell described by Lamarck, from the Paris-basin Eocene, under the name of 

 Biitiinus terebellaius, which agrees with it almost exactly in form, but differs 

 in being considerably larger, in having two or three more whorls, and in 

 being perforated or slightly umbilicated. The resemblance between these 

 shells is, however, not due to any near affinities between them, as they prob- 

 ably belong to widely distinct genera. 



Locality and position. — Thirty miles below Fort Clarke, Dakota; from 

 the summit of Square Butte. Fort Union Fresh- and Brackish-water Lignite 

 group ; probably Eocene. 



Hydi'obia subconica, Meek. 



Shell of medium size, conoid-subovate ; volutions about F, =- "■ 



five, slightly convex, or almost flattened to the slope of the 

 spire, last one rather large, and prominent or subangular I 



around the middle, and somewhat produced below ; suture Hydrobia subcomca. 

 not strongly impressed ; aperture less than the length of the spire, rhombic- 

 oval, angular above, and subangular and a little effuse below ; umbilical region 

 excavated and provided with a small perforation ; surface smooth. 



Length, 0.25 inch; breadth, 0.16 inch; length of aperture, 0.12 inch, 

 breadth of same, 0.U8 inch ; slopes of spire nearly straight, with a diverg- 

 ence from the apex of about 40°. 



This little shell is rather more nearly related to the last than to any of 

 our other Upper Missouri forms, but will be at once distinguished by its 

 shorter spire, broader body-whorl, smaller number of volutions, propor- 

 tionally larger aperture, perforated axis, &c. Its reference to the genus 

 Hydrobia is only provisional. 



Locality and position. — Mouth of Judith River, Montana; from the 

 Judith River group ; probably Eocene. 



Hydrobia? culimoidcs, Meek. 



Shell small, subterete ; spire elongate-conical, Fi s- 78 - 



acute at the apex ; volutions seven to eight in mature 

 examples, very slightly convex or flattened nearly on a 

 line with the slopes of the spire ; last one moderate and 



Hy&rdbia ? eulimoides. 



prominently rounded, or obscurely subangular around 



the middle ; suture linear, but rather deep; aperture ovate, angular above, 



narrowly rounded below, and equaling rather more than one-third the entire 



