53(3 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



3. planoebella, Haldeman. 



Shell with whorls few, and aperture campanulate. — {Planorbis 

 campanulatus, Say.) 



4. taphius, H. and A. Adams. 



Shell ventricose, somewhat irregular; whorls rounded on the 

 outer side, prominent or subangular around the rather small, deeply- 

 excavated umbilicus of the left side; aperture large, obovate, straight 

 within. — {Planorbis Andecolus, d'Orbigny.) 



5. menetits, H. and A. Adams (rr Anisus, Beck, not Fitz.). 



Shell depressed, volutions increasing rapidly in size; aperture 

 suboval ; periphery more or less angular. — (P. angulatus, Phil.) 



6. anisus, Fitzinger (= Tropidiscus, Stein.). 



Shell strongly depressed: volutions very numerous; periphery 

 angular. — (P. carinatus, Midler.) 



7. bathyomphalus, Agassiz (rz Spirorbis, Swainson, not Lamarck). 



Shell discoid, rounded on the periphery: whorls numerous, 

 closely embracing on the left side, exposed on the right ; aperture 

 narrow, crescentic ; umbilical cavity on the left side narrow and 

 profound. — {Helix contractu, Linn.) 



8. gyraulus, Agassiz (— Nautilina, Stein.). 



Shell discoid, slightly convex on the right side, flat or broadly 

 concave on the left ; volutions few, rapidly increasing in size, obliquely 

 depressed, but not angular. — (P. albus, Midler.) 



So far as at present known, the genus Planorbis seems to have been first 

 introduced during the Liassic epoch; it is also known to have been repre- 

 sented during the deposition of the Wealden formation. Many species have 

 likewise been found in the fresh-water Tertiary deposits of various countries ; 

 while the genus seems to attain its greatest development at the present time, 

 and is widely distributed, particularly in northern temperate regions. 



Planorbis convolutus, M. & H 



Plate 42, figs. 12, a, b. 

 Planorbis convolutus, Meek and Hayden (1856), Proceed. Acad. Nat. Sci. Philad., VIII, 120. 



Shell attaining a large size, discoidal, the whorls being all coiled very 

 nearly in the same, plane; upper and lower sides moderately and nearly 

 equally concave, and exhibiting about four-fifths of each whorl ; volutions 



