534 UNITED STATE3 GEOLOGICAL SURVEY OF TOE TERRITORIES. 



Subgenus PLEUROLIMNJEA, Meek. 

 L, i in ii ae a tenuit'ostati), M. & H. 



Plate 44, figs. 13, a, b, c. 



Limncea tenuicosiata, Meek and Hayden (1856), Proceed. Acad. Nat. Sci. Philad., 119. 

 Limncea (Acella) tcnuicostata, Meek and Hayden (1860), Proceed. Acad. Nat. Sci. Philad., 4:U. 



Shell very small, slender, thin, and fragile ; spire turreted, acute, about 

 equaling the length of the aperture ; volutions four to four and a half, convex, 

 increasing rather rapidly in length, but not in breadth; last one not ventri- 

 cose, narrowing and produced below; suture deep; surface-costse regularly 

 disposed, a little flexuous, arranged parallel to the minute striae of growth, 

 and numbering about eight to twelve on the last turn. 



Length, 0.30 inch; breadth, 0.0S inch: apical angle regular, divergence 

 about 21°. 



This species will be distinguished at a glance from all the others known 

 in our Upper Missouri rocks. Indeed, I am not acquainted with any other 

 form-, either uiiii'ii: recent or fossil shells, with which it is liable to be con- 

 founded. Its most marked features are its small size, slender form, and 

 costate surface. With the exception of the last-mentioned character, it is 

 evidently related to Limncea subulata of Sowerby, which, however, is 

 described as having the surface smooth. 



It is possible that a distinct genus should be established for the recep- 

 tion of this little shell, though I have concluded to ranee it provisionally as 

 the type of a subgenus under Limncea. 



Locality and position. — Three miles below Fort Union ; from the Fort 

 Union Lignite group, probably Lower Eocene. 



Genus PLANORBIS, Mtiller. 



Synon.—Planorbis, Muller" (177<>), Venn. Terrest. et Fluv., 152; and Zool. Pan., '238.— Brag. (1789), 

 Encyc. Meth., I, xvi.— Cuvier (1798), Tab. FJem.— Drap. (1805), Hist., 15, 28, and 42.— 

 Monti'. (1810), Conch. Syst., II, 270.— J. Sowerby (1810), Miu. Couch., tab., 140.— 

 Goldf. (1820), Hdb. Zool., 659.— -Blainv, (1824), Diet. Sci. Nat., XXXII, 244; and (1825) 

 Malac, 450; also (1826) Diet. Sci. Nat., XL., 222— Flein. (1828), Hist, Brit. An., 277.— 

 Gray (1^40), Tnrton's Man. (2d ed.),25C.— Desbayes (1824), Coq. Foss. Env. Paris, II, 

 81 : and il~r.li Suppl. to same, II. 734.— Fleming (1828), Hist. Brit. An., 277.— II. aid 

 i. Adams (1855), Genera Recent Moll., II, 260.— Meek and Haydeu (1865), Palajont. 

 Upper Missouri, 106; and of many others, but not Lain., 1799. 

 Vortex, Humphrey (179si. Mas. Col. (2d ed., 58); Dot (Him. 1815, nor Beck, 1837. 

 Anisus, Fitzinger (1833), Verz., III.— II. and A. Adams (1856), Genera of Recent Moll., II, 263 

 i as a subgenus nuder Plftorbis). 



* Guttard first described and named this genus in L75G; and he is often cited as its author. 

 According to Ibe rule thai 1 have adopted, bowever, I do not cite to authors dating before the intro- 

 duction of the binomial system of nomenclature, as the founders of genera. 



