386 GEOLOGY OF THE BLACK HILLS. 



tinuous curve with the antero-basal and basal margins ; posterior basal 

 margin more sharply rounded, and extended obliquely backward; posterior 

 wing large, scarcely pointed at the extremity, and separated from the body 

 of the shell below by a very broad shallow sinus. 



Surface marked by closely arranged, rounded, radiating ribs, separated 

 by flattened interspaces of twice their width, and increased in number by 

 interstitial addition ; about six or seven of the ribs occupy a space of a fifth 

 of an inch at the margin of the largest valve seen, which is about one inch 

 in its extreme height. The radii are crossed by very fine concentric lines 

 of growth, but so fine as to scarcely form a feature of the shell except 

 when viewed by the aid of a lens. Right valve of the shell only recog- 

 nized by a few fragments associated with a large number of left valves, on 

 the same block, and is apparently nearly or quite smooth. 



Formation and locality. — In hard concretionary limestone of the Fort 

 Pierre Group, on Sage Creek, Black Hills. 



PTERIA (PSEUDOPTERIA) FIBROSA. 



Plate 7. fig. 5. 



Avicula ? fibrosa M. & H. (1856), Proc. Acad. Nat, Sci. Phil., p. 86. 



Pholadomya fibrosa M. & H., ibid, p. 286. 



Pinna fibrosa Meek, Smithsonian Check List Invert. Foss., p. 9. 



Avicula (Pseudopteria) fibrosa M. & H., Sixth An. Sept. U. S. Geol. Snrv. Terr., p. 489. 



Pteria (Pseudopteria) fibrosa Meek, Pal. IT. S. Geol. Snrv. Terr., p. 36, PI. 17, fig. 17. 



A single very imperfect example of a left valve of this species has 

 been observed among the collections from the top of the gray shales of 

 No. 4, on the Cheyenne River, near Rapid Creek. The surface is much 

 exfoliated and the nacreous character of the shell destroyed. The shell is 

 very oblique, the axis of the body forming with the hinge line an angle of 

 about forty-five degrees. The valve is quite ventricose toward the beak, 

 but becomes compressed and flattened below. Anterior wing small and 

 obtusely pointed ; surface of the shell marked by rather strong concentric 

 undulations, and also by coarse, irregular, and unequal radiating folds of 

 the substance, which are not true radii, but simply foldings of the shell. It 

 may be readily distinguished from P. (P.) suhlcevis herein described by 

 its larger size, greater obliquity, and by the surface folds. 



