112 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



Voldia. \ onl i iciiMi. H. & M 



Plate 15, tigs. 5, a, h. 



yncnla vantricosa, nail ami Meek (1804), Mem. Am. Acad. Arts and Sci. (u. s.), V, 38£>, pi. 1, tig. 11 (not 



N. veutricosa, Hinds, 1843). 

 Toldia vcntricosa, Meek and Hayden (1860), Proceed. Acad. Nat. Sci. Philad., XII, 429. 



Shell very small, transversely ovate, ventricose in the middle, and 

 compressed at the extremities ; anterior end rather narrowly rounded, most 

 prominent a little above the middle; posterior side more contracted and 

 compressod, subangular at the extremity ; base prominent and rounded ; 

 beaks elevated, and located slightly in advance of the middle; surface marked 

 by regular concentric ridges, which are most distinct on the middle of the 

 valves, and obscure near the extremities ; sometimes a broad faint depression 

 extends from near the posterior side of each beak, toward the lower part of 

 the posterior extremity; hinge-line declining a little from the beaks, and 

 comparatively strongly crenate. 



Length, from 0.14 lo 0.1G inch; height, about 0.10 inch; convexity, a 

 little less than the height. 



The similarity between the form and other characters of this little shell 

 and Yoldia scitula, leaves very little room for doubt in referring it to the same 

 genus ; though none of the specimens yet examined show the pallial line 

 very distinctly. 



Locality and position. — The specimens first described and figured by 

 Professor Hall and the writer, were collected by Dr. Hayden in 1853, on Sage 

 Creek, from the upper beds of the Fort Pierre group. The only specimens 

 since seen, are from the South Fork of Clieyenne River, where they occur in 

 the same geological position. 



UNIONIDiE. 



Genus MARGARITANA, Schumacher. 



Synon.—Margaritana,Schum. (1817), Essai, 123.— Lea (1838), Trans. Am. Philos. Soc, VI (u. s.), 42 ; and 

 Observ. II, 118 (as a subgenus of Margaron).— Swaiuson (1840), Malac, 288 and 382. — 

 Agassiz (1852), Arcli. f. Nat., I, 314. 



Alasmidonta, Say (1818), Jour. Philad. Acad. Nat. Sci., I, 459.— Raf. (1620), Monog. Bivalves Ohio. 



Alasmodonta, Say (1819), Nicholson's Eucyc, 3d Am. ed., * ; and in Am. Conch. — Feruss. (1819), 

 Tab. Syst.— Barnes (1823), Am. Jour. Sci., VI, pi. 21.-Meuke (1830), Synou., 2d 

 ed., 106. — H. and A. Adams (1857), Gen. Recent Moll., II, 499 (as a subgenus of Baphia). 



A lasmodon, Thomas (18^0), Ann. Phil., XVI, 110.— Fleming (1828), Biit. An., 417.— Gray (1840), 

 Turton's Mai;., 2d ed., 293— Swainson (1840), Malac. 382. 



Alaamadonia, Say (1836). Catalogue, 23. 



