Logan.] The Invertebrates, Benton Group. 461 



Rostellites ambigua ? Stanton. Plate c, figs. 2-5. 



Rostellites ambigua Stanton, 1893, Bull. U. S. Geol. Surv. No. 106, p. 156, 

 pi. xxxin, ff. 8-10. 



Original description : "Shell rather large size, spindle-shaped ; 

 spire elevated and rather slender, somewhat more than half the 

 length of the aperture ; whorls about seven in number, moder- 

 ately convex, the last one very large ; suture distinct, more or 

 less channeled. Below the rounded border of the suture the 

 whorl is slightly constricted or flattened. Surface ornamented 

 by numerous closely arranged revolving lines and by larger 

 curved transverse costse that are less conspicuous on the body 

 whorl than on the spire, and that have a tendency to form 

 nodes just below the suture. The aperture narrow, prolonged 

 into a rather short canal, and with a short posterior, notch. 

 Outer lip thin, obscurely Urate within, and slightly re flexed ; 

 inner lip with a very thin callus. The pillar is slightly arched, 

 with two distinct folds, the anterior of which is the stronger, 

 and faint indications of two others behind them. Length of an 

 average specimen, seventy-seven mm. ; greatest breadth, twenty- 

 four mm. Other specimens show that the species attained a 

 considerably greater size and sometimes had a more robust 

 form." 



The above-described species occurs in the Septaria horizon of 

 the Fort Benton formation, where it is associated with R. 

 willistonii. 



Rostellites willistonii, n. sp. Plate cxx, fig. 3. 



Shell rather large, spindle-shaped; spire passing gradually 

 to a point, equally as long as the aperture ; whorls six in num- 

 ber, the body whorl being much larger than the first in the 

 spire. Whorls slightly constricted both above and below the 

 suture, which has a slightly raised border, which is somewhat 

 rounded. Suture not especially distinct. Surface marked by 

 parallel revolving lines and by curved, prominent costae, form- 

 ing oblong nodes or ridges on each whorl, but more prominent 

 ones on the body whorl than on the spire. Nodes decreasing in 

 prominence as the apex is approached. Aperture inversely 

 conical, prolonged into a moderately long canal. 



