36 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



Hind's collections, from south branch of the Saskatchewan, British America. 

 It is an abundant species, and was apparently of gregarious habits. 



Pteria (Pseudoptera) fibrosa, M. & H. 



Plate 17, figs. 17. a, !>, c, d. 



• 



Avicula ? fibrosa, Meek ami Hayden (1850), Proceed. Acad. Nat. Sci. Philad., 86. 



Pholadomya fibrosa, Meek and Hayden (1850), ib., 1280. 



Pinna fibrosa, Meek (1804), Smithsonian Check-List Cret. Fossils N.Am., 9. 



Avieula (Pseudoptera) fibrosa, Meek (1873), Hayden's Sixth Anu. Eep. U. S. Geol. Surv. of the Terr., 489. 



Shell thin, obliquely subovate, nearly or quite equivalve, both valves 

 being quite gibbous from the beaks obliquely backward and downward along 

 the central and anterior regions; hinge-margin very short and compressed; 

 posterior margin sloping backward and downward, with a slightly convex 

 outline, from the posterior end of the hinge into the rounded posterior basal 

 margin ; anterior margin truncated obliquely backward and downward from 

 the beaks, with a nearly straight or slightly concave outline; that of each 

 valve being curved rather abruptly inward toward the other, and apparently 

 gaping for the passage of the byssus, though there are no traces of a byssal 

 sinus in either valve; beaks gibbous, very oblique, pointed, incurved with a 

 forward obliquity, and very nearly if not quite terminal; posterior alation 

 obsolete, or only distinguishable by its compression from the swell of the 

 umbones, very obtusely angular, or rounding into the posterior margin 

 behind, from which it is not separated by the slightest sinuosity; anterior 

 wing apparently entirely wanting, surface ornamented by distinct, rounded, 

 more or less irregular, bifurcating, radiating costse, generally wider than the 

 furrows between, and crossed by regular, small, concentric ridges, which divide 

 them into little, obscure, node-like prominences. 



Length from the points of the beaks to the posterior basal extremity, 

 1.42 inches; greatest breadth at right angles to the longest diameter, 1 inch ; 

 convexity, about 0.35 inch. 



The radiating costse mentioned in the foregoing description are gen- 

 erally confined to the convex portion of the valves, being obsolete on the 

 compressed posterior dorsal region. Owing to the thinness of the shell, all 

 of the stronger surface-markings are well defined on internal casts. Some 

 specimens retaining portions of the shell show a few distant, subimbricating, 

 concentric lines, arranged one on each of the concentric ridges. In no 

 instance have any traces of muscular impressions been seen on any of the 

 internal rusts, or in tin 1 interior of the shell itself: and very little is vet 



