92 THE CHICAGO ACADEMY OF SCIENCES. 



Conch., p. 506, 1883. TRYON, Struct, and Syst. Conch., Ill, p. 101, 1884. 

 WHITEAVES, Geol. & Nat. Hist. Surv. Canada, Cont. Can. Paleont., I, pt. 1, 

 p. 13, 1885. TYRRELL, Rep. Geol. & Nat. Hist. Surv. Can., 1.886, n. ser. II, p. 

 68 E, p. 136, E, 1887. BOYLE, Bull. U. S. Geol. Surv., no., 102, p. 168, 1893. 



Limncea (Pleurolimncea) tenuicostata CONRAD, Smith. Check-list, p. 9,1886. 

 MEEK, U. S. Geol. Surv. Terr., IX, p. 534, pi. 44, fig. 13, 1876. WHITE, Bull. 

 U. S. Geol. & Geog. Surv. Terr., Ill, p. 613, 1877; Amer. Journ. Sci. & Arts, iii, 

 XX, p. 45, 1880; An. Mag. N. H., v, VI, p. 248, 1880; An. Rep. U. S. Geol. & 

 Geog. Surv. Terr., XII, p. 84, 1883. MARCOU, Proc. Nat. Mus., VIII, p. 330, 

 1885. WHITE, Bull. U. S. Geol. Surv., no. 128, p. 75, 1895. SCHUCHERT, Bull. 

 U. S. Nat. Mus. 53, p. 357, 1905. BAKER, Science, N. S., XXVII, p. 943, 1908. 



Lymnaa (Pleurolimn&a) tenuicostata BALL, Alaska Mollusks, p. 64, 1905. 



Acclla sp. WHITE, Proc. U. S. Nat. Mus., HI, p. 160, 1880. 



Acella micronema WHITE, Bull. U. S. Geol. Surv., 34, pp. 11, 22, pi. 2, 

 fig. 14, 1886. SCHUCHERT, Bull. U. S. Nat. Mus., 53, p. 20, 1905. 



''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 ventricose, narrowing and produced below; suture deep; 

 surface-costae 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, 0.08 inch; apical angle regular, di- 

 vergence about 21" (Meek). 



TYPE: Nat. Mus. no. 12488, tcnuicosta; no. 20066, micronema. 



HORIZON : Laramie formation, Cretaceous Period. 



Fort Union formation, Lower Eocene Period. 

 Wasatch formation, Lower Eocene Period. 



LOCALITY: Cretaceous (Laramie). Mouth of the Blind Man 

 River, Township 39, Range 27, west of the Fourth Principal Meridian ; 

 Blind Man or Paskapoo River ; three-quarters of a mile upstream from 

 the Red Deer a six-inch seam of coal crops out forty feet above the 

 bed of the stream, underlain by a thin bed of marl sand, in which the 

 fossils occur (Alberta). 



Lower Eocene (Fort Union). Three miles below Fort Union 

 (Fort Buford), Williams County, North Dakota. 



Lower Eocene (Wasatch). Wales, Sanpete County, Utah, near 

 junction of Wasatch and Green River formations; three miles east of 

 Table Rock station, southern Wyoming. 



REMARKS: "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 among recent or fossil shells, 

 with which it is liable to be confounded. Its most marked features 



