106 PALEONTOLOGY OF THE UPPER MISSOURI. 



In their habits, those mollusks all agree in being inhabitants of fresh water. 

 They are true air-breathers, being compelled to come to the surface occasionally for 

 that purpose. They are widely distributed in almost all parts of the world where 

 ponds, streams, and other bodies of fresh water exist, and feed upon confervas and 

 other aquatic vegetation. 



Subfamily PLANORBINtE (p. 104). 

 Genus PLANORBIS, Muller. 



Synon.—Planorbis (part), MiJUEK, Verm. Terest. et Fluv. 1774, 152 ; Zool. Dan. 1776, 23S.— BRno. Encyc. 1789, I, 

 xvi— CoviER, Tab. Elem. 1798 (not Ptanorbis, Lamarck, Prodr. 1799, 76 ; nor 1801, Syst. An. 93). 



Orbis, ScHROT. Jour. F. d. L. d. Ill, 1776, 10 (not Lea, 1833). 



Vortex, Hdmphrey, Mus. Col. 1797 (58, sec. ed.). 



Anisus, FiTZ. Verz. 1833, 111. 



? Bathyomphalus, Agassiz, Catal. 1837, 20. 



Helisoma, SwAissoN, Malac. 1840, 337. 



Spirorbis, Swainson, ib. (not Lamarck, 1815). 



Planorbina, Haldeman, Fresh-water Univ. U. S. 1842, 14. 



Planorbella, Haldeman, ib. 1842. • 



? Gyraulus, Agassiz, Nouv. Mem. Soc. Helv. I, 1837. 



PlauodiscHS, Stein, * * * 1843. 



« Taphius, H. & A. Adams, Genera Kecent Mol. II, 1856, 264. 



Menetus, H. & A. Adams, ib. 

 Elym. — Planus, flat ; orbis, an orb. 

 Type. — Helix cornea, Linnaeus. 



Shell dextral, or sinistral '?^ discoidal or subdiscoidal, the whorls being nearly or 

 quite on the same plane; right side generally flat, or sometimes either a little 

 elevated or concave ; left side more or less excavated ; volutions rounded, com- 

 pressed, or angular; aperture crescentic or suboval, sometimes dilated; peristome 

 thin, incomplete, right margin projecting. 



The typical forms of tliis genus have the shell much depressed, and the volutions 

 numerous, rounded or without angles, and visible on both sides ; while the mouth 

 is not dilated. As above defined, hoAvever, it is made also to include several subor- 

 dinate groups which depart more or less from the typical species, though generally 

 placed here by conchologists. Some of these types should probably stand as distinct 

 genera, but as it is scarcely practicable, in Palaeontology at least, always to distin- 

 guish between them, we have preferred to define the genus in its widest sense. 

 The subordinate groups, however, not agreeing exactly with the typical forms, may 

 be characterized as follows: — 



1. Planorbella, Haldeman. 



Shell with few whorls, which are usually angular on the left side ; aperture distinctly expanded, or bell- 

 shaped. 

 Type. — Ptanorbis campanulatus, Sat. 



' Conchologists generally regard these depressed shells as being dextral ; but O. A. L. Morch 

 offers some apparently good reasons for viewing them as properly sinistral forms (Conch. Jour, xi., 

 2d Ser. 235). This conclusion seems to be sustained by the form of the young of some American 

 species, one of which was described by DeKay as a truncated Phyaa. On the other hand, however, 

 monstrosities of some foreign species with an elevated spire, are generally dextral. 



