72 PAL-E ONTOLOGY OF THE UPPER MISSOURI. 



characters in which it resembles some Jurassic species, but it seems, so far as can 

 be seen, to be also quite as nearly like others of various ages, even amongst the 

 Palaeozoic forms. 



Those we have examined are small, subrhomboidal, moderately convex, and have 

 on the dorsal or smaller valve (avc have not seen the other) about fifteen simple, 

 obtusely angular plications, four or five of which are raised near the front, so as to 

 form a moderately prominent, rather flattened mesial fold. The surface is also 

 marked by fine, very obscure lines of growth, whicli are deflected upwards in 

 crossing the pUcations, near the front. 



Locality and position. — Southwest base of Black Hills (Jurassic), Dakota Territory. 

 (Type 319.) 



Class LAMELLIBRA^CHIATA, 



Family OSTREID^. 



Shell more or less irregular, inequivalve, slightly inequilateral, lying 

 upon, and generally attached by, the left valve. Beaks straight or 

 curved ; hinge edentulous ; ligament subinternal ; muscular impression 

 nearly central, or behind the centre ; pallial line obscure, simple. 



Animal with its mantle entirely open, and provided with double, 

 fringed edges, which are without distinct ocelli ; foot obsolete ; gills 

 crescent-shaped, and separated from the palpi ; labial appendages tri- 

 angular, connected around the mouth by a plain membrane. 



This famUy probably only embraces the three closely allied groups — Ostrea, 

 Gryphaxi, and Exogyra ; the latter two of which are entirely extinct, excepting a 

 single species apparently of Qryphaia. The curious extinct genus Eligmus, of 

 Deslongchamps, is also placed here by some authors, but we are not well enough 

 acquainted with these pecidiar shells to express an opinion in regard to their 

 afiinities. 



Genus OSTREA, Linn^us. 



Si/non. — Ostracites, Ostreites, Limnnslracites, Ostrcum, &o. (sp.), Llhwyd, Klein, ami other pre-Linn;eau authors. 



Oslrea, LiNN^DS, Syst. Nat. ed. 10th, 1758, 696. — 0. F. Mdller, Prodr. Zool. Dan. xxxi. — Brcg. Encyc. 

 Meth. I, xiii.— Lamk. Syst. 1801, 132, &c. 



Pcloris, PoLi, Test. Utr. Sic. 1791, 33. 



Peloriderma, PoLi, ib. II, 255. 



Lopka, BoLTEN, Mus. Col. 1798 (2d ed. 117). 



Alectryonia, FiscuER DE Waldheim, Mus. Dem. 1807, and Bull. Mosc. VIII, 1835. — Ciienu, Man. Conch. II, 

 1862, 167. 



Dendrostrea, SwAiNSON, Malacol. 1840, p. 389.— G. B. Sowerby, Conch. Man. 2d ed. 137. 



Etym oa-Tfsov, an Oyster. 



Type. — Ostrea edulisy Linn. 



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

 surface often ornamented by radiating plications, and more or less imbricating, 

 or distinct marks of growth. Upper valve flat or concave ; lower valve convex, 

 and having a prominent beak. Ligament occupying a pit or groove in the cardi- 

 nal area of each valve. MusciUar impression subcentral. 



