TO YELLOWSTONE NATIONAL PARK. 145 
between the points of greatest length; dorsal margin less regular than the basal, slightly con- 
tracted behind the beaks; auterior side somewhat rapidly sloping for two-thirds of the distance 
between the beaks and anterior extremity ; extremities sharply rounded, a little less abruptly 
above than below the middle of the height. Beaks of moderate size, rather broad, slightly pro- 
jecting above the cardinal line, that of the right valve the largest and extending beyond the left. 
External ligament small, prominent, and situated close behind the beaks. 
Surface of the valves moderately convex, and apparently a little bent in an anterior and pos- 
terior direction ; the left valve being the most convex. (This is the opposite from what is usually 
the case.) Valves marked by distinct but irregular and somewhat crowded concentric undulations, 
and also by a slightly depressed, oblique, somewhat curving sulcus extending from behind the 
beaks to the postero-basal border, which it scarcely modifies. Internal features and hinge-structure 
unknown. 
The shell bears considerable resemblauce to Thracia Prouti Meek and Hayden (= Tellina Prout 
M. & H., Proc. A. N.S. Phil., vol. 8, p. 82), but is less contracted posterior to the beaks, and the 
dorsal margin slopes more rapidly anteriorly, the shell being less full and rounded on this part ; 
the beaks are also larger, and project above the cardinal line more than in that one; the basal line 
is also more regularly curved, that one rounding upward more strongly in front and less so behind, 
giving a straighter postero-basal margin. 
Formation and locality—tIn rocks of Cretaceous age at the mouth of the Judith River, Mon. 
tana, which overlie the Fort Pierre shales of that locality. 
Genus VANIKOROPSIS, Meek. 
VANIKOROPSIS TOUMEYANA. 
Plate 2, figs. 11-13. 
Natica Toumeyana, M. & H., Proc. A. N.S. Phil., vol. viii, p. 270, 1856. 
Naticopsis ? Towmeyana, M. & H., ib., vol. xii, p. 423.—Meek, Smithsonian Check-List, Ivvert. 
Foss., p. 18, 1864. 
"anikoropsis Toumeyana, Meek, Pal. U. 8. Geol. Surv. Territ., p. 332, pl. 39, fig. 2. 
Shell rather large, naticoid in form, subglobose and a little oblique, composed of about four 
very ventricose, but not inflated volutions ; spire short, depressed, conical, the slope of the spire 
inclosing an angle of about one hundred and five degrees ; suture-line deep and well pronounced ; 
body-volution forming more than two-thirds of the entire height of the shell; aperture broadly 
oval, rounded, and very slightly extended below, a little straightened on the columellar side, and 
slightly modified above by the preceding volution ; columellar lip thickened and spreading on the 
body of the preceding volution, and covering but not concealing the umbilicus, or forming a true 
callus. Umbilicus small and deep. 
Surface of the two outer volutions marked by strong, transverse undulations, or ridges, par- 
allel to the margin of the aperture, and numbering about fifteen on the outer whorl; also, by 
coarse, revolving bands which cross the undulations and have slightly flattened interspaces ; four 
of the bands occupying the space of about one-fourth of an inch on the middle of the outer 
volution. Substance of the shell very thick and solid. 
When describing this species, we had supposed it to be entirely new, not having recognized it 
in the description of the imperfect individual used by Mr. Meek; but, on seeing his figure above 
cited, we suspect it may only be a more strongly marked individual of that species, and, although 
no direct comparison has been made, we do not hesitate to consider it in that light. It differs, 
however, in being more elevated and in the stronger vertical folds. 
Formation and locality.—In beds of Cretaceous age overlying the Fort Pierre shales near the 
mouth of the Judith River, Montana. 
19 Ww 
