10 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



instead of subtrimcate anterior margin. It never attains one-fifth the size of 

 L. Rouliniana, d'Orbigny (Paleont. Fr. Terr. Cre"t., Brach., 10, pi. 490, 

 fig. 1), and has a much less pointed beak, as well as a more narrowly 

 rounded front margin. ' 



Locality and position. — Month of the Big Horn River; from the horizon 

 of the base of the Fox Hills group, where it was found associated with 

 Pieria (Oxytoma) Nebrascana. 



LAMELLIBRANCHIATA. 



MONOMYARIA. 



OSTREIDiE. 



Genus OSTKEA, Linnaeus. 



Synon. — Ostracites, Ostrailes, IAmnostracites, Ostreum, &c. (sp.), Llwyd, Klein, and other pre-Linntean 



authors. 

 Ostrea, Linnaeus (1758), Syst. Nat., 10th ed., 696.— O. F. Miiller (1776), Prodr. Zool. Dan., sxxi 



and 247.— Brug. (1789), Eucyc. Meth., I, xiii.— Lam. (1799), Prodr. 81; and Syst. (1801), 



132, &c. 

 Pcloris, Poli (1791), Test. Xltr. Sic, 33. 

 Peloriderma, Poli (1795), ib., 11,255. 

 Lopha, Bolten (1798), Mus. Coll., 2d ed., 117.— H. and A. Adams (1858), Genera Recent Moll., II, 



569 (as subgen. under Ostrea). 

 Alectryonia, Fischer do Waldheim (1807), Mus. Dem. ; and (1835), Bull. Mosc., VIII. — Chenu 



(1862), Man. Conch., II, 167.— Stoliczka (1871), Pal. Iudica, III, 454 (sec. Ostrea). 

 Dciulrostrea, Swainsou (1840), Malacol., 387. — G. B. Sowerby (1839), Couch. Man., 137. 

 Gryphceostrea, Conrad (1865,) Am. Jour. Conch., I, 15 ; (subgenus). 



Etym. — darpcov, an oyster. 

 Type.— Ostrea edulis, Linn. 



Shell irregular, laminated, subnacreous, attached by the left or under 

 valve ; surface sometimes nearly smooth, but more frequently provided with 

 more or less prominent imbricating laminae and smaller marks of growth, or 

 plicated, and very rarely armed with projecting root-like processes. Upper 

 valve flat or concave, and often plane ; lower valve convex, and having a 

 prominent beak. Ligament occupying a mesial longitudinal furrow, extend- 

 ing to the beaks in a kind of cardinal area marked by transverse striae. Mus- 

 cular impression subcentral. 



Of the genus Ostrea, there are three sections or subgenera, that are even 

 viewed by some as forming distinct genera. These may be defined as 

 follows : 



