UNDOUBTED TERTIARY SPECIES. 189 



higher in proportion to breadth than the others, while more or less difference 

 in the elevation or depression of the spire is observable. The most marked 

 differences, however, are to be observed in the character of the surface- 

 markings. Yet, since proposing for it a dictinct name, more critical com- 

 parisons lead me to suspect that it may be only the young form, or the 

 inner volutions of the last. 



. Another form, which I described as C. Tryoni var. concava, in the Pro - 

 ceedings of the Academy, cited above, has the spire depressed below the 

 top of the body-whorl, so as to be a little concave. This may possibly be 

 a distinct species, but it agrees so nearly with the tjrpical C. Tryoni in 

 nearly all other respects that I am inclined to regard it as a mere variety 

 of this shell. It is represented by fig. 10 c of plate 17. 



Locality and position. — Fossil Hill, Kaw-soh Mountains, Nevada ; Tertiary. 



Genus PLANORBIS, Guttard. 

 Planorbis spectabilis, Meek. 



Plate 17, figs. 13, and 13 a, b, c, d, e,f. 



Planorbis spectabilis, Meek (1860), Proceed. Acad. Nat. Sci. Philad., XII, 312 ; and 

 (1876) in Col. Simpson's Report Expl. across the Great Basin of Utah, 366, pi. 

 V, figs. 7 a-d.* 



Shell rather large, discoidal, concave above and below, but more deeply 

 below than above; volutions about six, increasing gradually in size, gen- 

 erally a little higher than wide, rounded on the periphery, more narrowly 

 rounded below, and convex but more depressed above ; each about half 

 enveloped below, and near one-fourth above, by the siicceeding turn. Sur- 

 face and aperture unknown. 



Greatest breadth, 0.80 inch ; height, 0.26 inch ; section of outer volu- 

 tion, 0.23 inch wide by 0.26 inch in hight. 



Since describing this shell under the above name, I have suspected that 

 it may possibly be only a large variety, or more fully developed P. con- 

 vohdus, M. & H. It has much the same form ; but, as the types were found 

 at widely different localities, and apparently at different horizons, I do not 

 feel sure that they can be properly united under one specific name. 



Locality and position. — The specimen figured was found by Colonel 

 Simpson on Ham's Fork, Wyoming. 



♦These figures were drawn from better specimens than those here illustrated. 



