CEETAOEOUS FOSSILS. 409 



Genus YOLDIA Moller. . 

 YOLDIA EVANSI. 



Plate 11, fig. 1, 2. 



Nucula evansi M. & H., Proc. Acad. Nat. Sci. Phil., 1866, vol. 8, p. 84, and ext. p. 21. 



Leda evansi M. & H., Proc. Acad. Nat. Sci. Phil., 1860, May, p. 185. 



Leda (Yoldia) evansi M. & H., ib., 1860, p. 429. 



Yoldia evansi Meek, Smithsonian Check List Invert. Foss., p. 7. 



Yoldia evansi (M. & H.) Meek, Pal. U. S. Geol. Surv. Terr., p. Ill, PL 28, fig. 10. 



Shell small, transversely elongate-elliptical, as long again as high. 

 Valves depressed-convex, most ventricose on the umbones and becoming 

 somewhat flattened posteriorly ; beaks small and inconspicuous, located at 

 about one-third of the length from the anterior end ; hinge line straight 

 and distinctly marked by a depressed longitudinal groove on each valve ; 

 seen only in a cardinal view ; anterior end rounded ; posterior end trun- 

 cate ; longest near the cardinal line and receding below ; basal line very 

 broadly curved. 



Surface polished and marked by very fine lines of growth. Hinge 

 line occupied by a large number of small, angular teeth, about twice as 

 many on the posterior as on the anterior side of the beak ; muscular im- 

 pression and pallial line faintly marked, the latter rather deeply sinuate. 



The species may be easily recognized by the peculiar longitudinal 

 groove along the hinge line when the shell is preserved. 



Formation and locality. — In limestone of the Fort Pierre" Group, on Old 

 Woman Fork of the Cheyenne River, and near Rapid Creek, Black Hills. 



Lucimvm. 



Genus LUCINA Bruguiers. 

 LUCINA OCCIDENTALIS. 



Plate 11, fig. 19-21. 



Tcllina occidentalis Morton, Jour. Acad. Nat. Sci. Phil., vol. 8, PI. xi, fig. 3. 

 Lncina occidentalis (Morton) M. & H., Proc. Acad. Nat. Sci. Phil., Nov. 1856 ; Ext. p. 20. 

 Jjucina occidentalis (Morton) M. & H., Smithsonian Check List. Invert. Foss., p. 12. 

 Mold of Lucina % Owen, Geol. Eept. Iowa, Wis. and Minn., Tab. vii, fig. 8. 

 Lucina occidentalis (Morton) Meek, Pal. U. S. Geol. Surv. Terr., p. 134, PI. 17, fig. 4. 



