384 DESCRIPTIONS OF NETV SPECIES OF FOSSILS, 



distinguished by its oblique form and extended beaks, its ventricose character, and the 

 fibrous or striated structure of the interior, produced by exfoliation. It occurs more 

 commonly in the septaria, which furnish only casts, the shell adhering to the rock ou 

 breaking, while the entire specimens are only obtained from the clay. Length, 1.4 

 inches ; height, .97 inch ; width, .70 inch. 



Locality and Position. — Sage Creek, in the upper part of division No. 2 of Section. 



Pectunculus Siouxensis, n. sp. 

 Plate I. Fig. 12. 



Shell sub-orbicular (in the cast) ; beaks elevated, nearly central ; longer than high, 

 nearly convex ; anterior margin regularly rounded ; posterior margin somewhat 

 obliquely sub-truncated ; basal margin without crenulations ; cardinal margin curved 

 and marked by fine dividing crenulations ; posterior muscular impression strong. 



The specimen described is a cast preserving the form of the shell, and showing very 

 distinctly the crenulations of the cardinal margin. The external markings of the 

 shell are unknown. 



Locality and Position. — Mouth of Big Sioux, on the Missouri River, in a fer- 

 ruginous sandstone. Division No. 1 of Section. 



NUCULA SUBNASUTA, n. Sp. 

 Plate I. Fig. 10, a, J, c. 



Shell sub-elliptical, contracted towards the posterior extremity, somewhat ventricose 

 in the middle ; a shalloAV groove or depression extending obliquely from the beak to 

 the base of the shell, where it produces a slight indentation in the regular elliptic 

 curve of the basal margin ; escutcheon margined by a broad shallow groove, extending 

 from near the beak, and causing a faint emargination near the posterior extremity 

 above ; beaks nearly central, small and incurved ; shell marked by faint stria; or lines 

 of growth, and a few broader concentric undulations which give a scarcely perceptible 

 inequality to the surface; crenulations of the hinge line very fine. Length, .78 inch ; 

 height, .45 inch ; width, .36 inch. 



This neat little shell is sufficiently well marked to be readily distinguished, particu- 

 larly by the slight impression on the edges of the shell above and below near the 

 posterior extremity. The crenulations are fine and slender ; and in one specimen the 

 erosion of the shell exhibits the crenulated edge of a former hinge line, at some dis- 



