458 University of Kansas Geolog-ical Survey. 



This species occurs in a thin stratum of sandstone underlying 

 the Lincoln marble group of the Fort Benton formation. 



CalUsta {Aphrodina) tenuis H. & M. Plate xcix, fig. 8, 9. 



Cythera tenuis H. & M., 1856, Mem. Am. Acad. Sci., n. s., vol. v, p. 383, pi. i, 



' f . 5. 

 Meretrix tenuis M. & H., 1860, Proc. Acad. Nat. Sci. Phil., p. 185. 

 ? Dione tenuis Meek, 1861, Smiths. Check-List, Inv. Foss. of N. A., p. 18. 

 CalUsta (Aphrodina?) tenuis Meek, 1876, U. S. Geol. Surv. Terr., vol. ix, 



p. 188, pi. v, ff. la-d; Stanton, 1893, Bull. U. S. Geol. Surv., p. 109, pi. 



xxiv, ff. 7 and 8. 



Description : Shell equivalve, subcircular or subovate, moder- 

 ately thin ; beaks incurved slightly in specimen and placed in 

 the antero-central region ; anterior side of shell moderately 

 short, rounded below and moderately straight above ; posterior 

 side somewhat subtruncate ; basal margin short, forming a semi- 

 oval curve. Beaks smooth ; surface marked by fine concentric 

 lines of growth. Length, twenty-four mm. ; height, sixteen 

 mm. ; convexity, ten mm. 



Specimen was collected by the writer from the Septaria hori- 

 zon of the Fort Benton, in the vicinity of Williams butte, in 

 Mitchell county. It consists of the right and the left valve 

 united and firmly held together by an internal cast. For that 

 reason none of the internal characteristics can be noted, and its 

 classification is dependent upon its external features, which are 

 none too well preserved. Species of this and kindred genera 

 are not abundant in the Septaria horizon. 



Wactra emmonsii Meek. Plate xcn, fig. 11. 



? Hactra emmonsii Meek, 1877, U. S. Geol. Expl. 40th Parallel, vol. iv, pt. 



i, p. 153, pi. xv, f. 8. 

 Mactra emmonsii Stanton, 1892, Bull. U. S. Geol. Surv., p. 121, pi. xxvn, 



ff. 9-13. 



Revised description : Shell small, compressed, longer than 

 high, equilateral, anterior side in some specimens longer than 

 the posterior side ; anterior margin narrowly rounded below the 

 middle ; posterior border somewhat broader ; basal margin form- 

 ing a semielliptical curve ; dorsal margin sloping before and 

 behind the beaks, the anterior slope being greater, with a con- 

 cave outline ; beaks nearly central, or sometimes placed a little 



