182 PALEONTOLOGY. 



entire length; suture well defined; aperture subovate, narrowly rounded 

 below and angular above, scarcely equaling one-half the length of the shell ; 

 columella apparently with only a very small fold, and showing by the side 

 of the inner lip below, appearances of a small umbilical indentation. Sur- 

 face marked by fine, rather obscure lines of growth, with (on some speci- 

 mens) exceedingly faint traces of microscopical revolving striae. 



Length, 0.40 inch; breadth, 0.20 inch; apical angle convex, divergence 

 40°. < 



Among recent species, this may be compared with L. humilis, and 

 the several forms regarded by Mr. Binney as varieties of that species. It 

 is a decidedly more slender shell, with a less expanded aperture and less 

 developed inner lip than Mr. Binney's figure (p. 63, fig. 99, Land and 

 Fresh Water Shells) of an authentic specimen of that species. It agrees, 

 however, more nearly in form with some of those figured by Mr. Binney 

 as varieties of L. humilis, but still differs from them all in other details. 



Locality and position. — Same as last. By some oversight, in first describ- 

 ing this species, the locality was wrongly given as Ham's Fork, latitude 41° 

 40' north, longitude 110° 10' west. 



SPECIES OF UNDOUBTED TERTIAKY AGE. 



MOLLUSCA. 



LAMELLIBRANCHIATA. 



CYRENID^. 



Genus SPHiERIUM, Scopoli. 

 Sph^rium rugosum, Meek. 



Plate 16, figs. 2, 2 a, and 2 h. 



Sphcerium rugosum, Meek (1870), Proceed. Acad. Nat. Sci. Philad., Ap., 56. 



Shell above medium size, rather gibbous, thick, quadrato-suborbicular 

 in outline, the length being a little greater than the height; greatest con- 

 vexity slightly above the middle; anterior margin more or less regularly 

 rounded; base semi-elliptic or sub-semicircular in outline; posterior margin 



