366 EXPLOKATIONS ACROSS THE GREAT BASIN OF UTAH. 



like the vertical folds, are obscure or quite obsolete, while on other specimens they 

 are distinctly denned on all the volutions. 



In most cases, the whorls are very nearly flat, but those of other individuals are 

 more convex. It is possible that these two forms may belong to distinct species, but 

 there are so many intermediate gradations in this respect that I am inclined to regard 

 them as merely varieties of one species. 



There are several quite similar forms among our recent Melanians, such for in- 

 stance as Goniobasis comma, Conrad, and G. athleta of Anthony, from which, however, 

 this species will be readily distinguished by obvious characters. 



The specific name is given in honor of Capt. J. H. Simpson, Topographical En- 

 gineers, United States Army, commander of Utah Exploring Expeditions, &c. I am in 

 doubt in regard to the relations of this shell to one of the forms described by Professor 

 Hall in Fremont's Report. Indeed, from first to last, I have had, as it were, to grope 

 in the dark in regard to the fresh-water fossils described in that report, on account of 

 the brevity of the descriptions and unsatisfactory figures, together with the uncertainty 

 of the exact localities from which they were obtained. 



Locality and position.— hater Tertiary beds at Ham's Fork, northeast of Fort 

 Bridger, latitude 41° 40' north, longitude 110° 10' west. Probably Miocene. 

 Goniobasis arcta, Meek. 



Plate 5, fig. 5. 

 Melanin arcta, Meek (July, 1860), Proceed. Acad. Nat. Sci. Philad., XII, 314. 



Shell rather small, very slender, terete; volutions about twelve, flattened-convex, 

 increasing very gradually from the apex; suture distinctly defined; surface showing 

 an exceedingly slight tendency to develop moderately broad, rather distant, vertical 

 folds, with faint traces of small revolving striae; aperture ovate. 



Length, 0.56 inch; breadth, 0.17 inch; apical angle regular, divergence 15°. 



This shell I now rather regard as only a slender variety of the last-described spe- 

 cies; but it differs so much from all the specimens I have seen certainly belonging to 

 that variable shell, that, with the collections at hand for comparison, this cannot be 

 clearly demonstrated. It is as much as one-third to one-half narrower, and has two or 

 three whorls more than well-marked specimens of J/. Simpsoni of its own length; while 

 its whorls differ in being flattened more obliquely above. 



The lower part of each" whorl rounds abruptly into the suture below, so that the 

 most prominent part is generally just above the suture. This prominence is also con- 

 tinued around the middle of the body-whorl. 



Locality and position. — Same as last. 



Genus PLANORBIS, Miiller. 

 Planoebis spectabilis, Meek. 



Plate 5, fig. 7, a, 6, c, d. 

 Planorbis spectabilis, Meek (July, 1860), Proceed. Acad. Nat. Sci. Philad., XII, 314. 



Shell large, moderately compressed; upper side slightly convex, sometimes a lit- 

 tle concave in the middle; periphery rather narrowly rounded below the middle; vo- 

 lutions five and a half, increasing gradually in size, wider than high, depressed-convex, 

 and sloping a little outward above, distinctly convex below; about one-half of each 



