(304 UNITED STATES GEOLOGICAL SURVEY OE THE TERRITORIES. 



4. isidora, Ehrenb. (— Diastropha, Guilding). 



Shell ovate, with volutions ventricose; axis slightly umbilicated; 

 columella without a fold. — {Physa contorta, Mich.) 



5. costella, Dall. 



Shell rounded, with vertical undulations or costse. — {Physa 

 cosiata, Newcomb.) 



The existing species of this genus usually live in clear running streams, 

 or springs, though they also inhabit clear ponds. They are widely distrib- 

 uted, being found in North America, Europe, India, Southern Africa, the 

 Philippine Islands, &c. 



This genus seems to have been introduced near the close of the Oolitic 

 period, species being found in the Purbec beds of England. It also occurs 

 in the Wealden of Europe, and in the Tertiary rocks of this country and 

 Europe. Several very fine, large species are already known from the Ter- 

 tiary rocks of Wyoming. Our eastern coast Tertiary beds, being almost 

 exclusively of marine origin, have consecpientlyno species of this genus, so 



far as known. 



P li y s a secalina, E. & S. 



Plate 45, figs. 4, a, b. 



Physa secalina, Evans and Slmniard (1654), Proceed. Acad. Nat. >Sci. Pbilad., VI, 15G. 



Shell small, narrow-oval, or subelliptical ; volutions three and a half to 

 four, last one comparatively large, but not ventricose, composing about three- 

 fourths of the entire length; spire comparatively small ; suture moderately 

 distinct; aperture very narrow-subovate, acutely angular above, and narrowly 

 rounded below ; surface apparently smooth. 



Length, 0.17 inch; breadth, 0.08 inch. 



Locality and position. — Pinots' Creek, upper beds of the White River 

 Tertiary, Dakota Territory. 



HELICID^. 



Genus HELIX, Linn, (soe page 540). 

 Helix Leidyi, H & M 



Plate 15, figs. < , », h. 

 Helix Leidyi, Hall and Meek (1851), Meiu. Am. Acad. Arts and Sci. Boston, V (n. s.), 394, pi. 3, fig. 12. 



Shell subglobose, wider than high; spire rather prominent; volutions 

 moderately convex, las! one large and ventricose, rounded; suture distinct; 



