160 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



shore-deposit, into which brackish-water shells mighf readily be carried and 

 deposited along with marine forms. 



Locality and position. — Month of the Big Sioux River, on the Missouri ; 

 in the Dakota group of the Upper Missouri Cretaceous series. 



Genus COKBICULA, Miihlfeldt. 



Synon— Corbicula, Miihlfeldt (1811), Berliner Mag., V, 56.— Blainville (1825), Malac, 552 (as section 

 Cyelas).— Rang (1829), Man., 313 (as a subgenus of Cyrena.)— Nyst (1838), Bull. Acad. 

 Brnxelles, * » .— H. and A. Adams (1858), Genera Recent Moll., II, 147.— Prime (I860), 

 Proceed. Acad. Nat. Sci. Philad., 267; and Annals Lyceum N. H., N. Y., VII; also 

 (1863), Cat. Corbiculidce, 3; and (1864), Notes ou Species of the Corbiculidce, with 

 figures, Ann. Lye. N. H., N. Y., VIII, 57. 

 Cyrcua (part), of many, but not of Lamarck, as now restricted. 



Etym. — Corbicula, dim. of corbis, a twig-basket. 

 Examp. — C. fluminea, Miiller (sp.). 



Shell usually solid, varying according to the species and sections from 

 subtrigonal to suborbicular, or transversely subovate, and presenting a more 

 or less cordate outline, as seen in a front or posterior view ; valves closed ; 

 surface often concentrically furrowed; epidermis polished; hinge with three 

 more or less bifid, diverging, cardinal teeth in each valve, the front in the 

 right valve, and the posterior in the left, smallest, or sometimes nearly obso- 

 lete; lateral teeth much elongated, narrow, and finely striated transversely; 

 pallial line with a more or less denned, sometimes very nearly obsolete, sinus. 



It is epiite probable that a critical study of the numerous extinct species 

 that have been described under the general name Cyrena, by those who are 

 not very particular in regard to generic distinctions, would bring to light 

 sufficient reasons for the separation from that genus and Corbicula, of several 

 groups, either holding the rank of distinct genera or subgenera. Not having 

 at hand, however, the necessary materials for such a stud}', no attempt will 

 be made here in that direction, beyond the notice of two sections that have 

 already been proposed by the writer as subgenera under Corbicula. These 

 were founded on species from brackish- water deposits in Wyoming Territory, 

 belonging to the very latest of the Cretaceous or the oldest of the Eocene 

 Tertiary age, and were published in Dr. Hayden's lveport of 1872, under the 

 names Veloritina and Leptestkes. These sections may be distinguished from 

 each other, and from the typical forms of Corbicula, as stated below: 



