INVERTEBRATE PALEONTOLOGY. 535 



Bathyomphalus, Agassi/,. MS., Cbarp. ( 1837 ), C:it:il.. 20 (as a subgenus under Planorbis). — Meek and 

 Hayden (1865), Pal aeon t. Upper Missouri, 105 (used as a subgenus under Planorbis). 



fl«Ksoma,«Swainson (1840), Malae., 337.— H. aud A. Adams (1856), Genera Reeenl Moll., II, 262 

 (as a subgenus under Planorbis). 



Spirorbis, Svvaiuson (1840), Malac.,337. — H. and A. Adams (1856), Genera Recent Moll. 263 (as a 

 subgenus under Planorbis); not Spirorbis of Lamarck. 



Planorbulina, Haldeman ( 1842), Fresh- vi ater Univalyes U. S., 14. 



Planorbella, Haldeman (1842), ib.— H. aud A. Adams (1856), Genera Recent Moll., II, 261 (as 

 a subgenus under Planorbis). 



Nautilina, stein. (1850), iu Schneck. et Mus. Berk, 80.— H. and A. Adams (1856), Genera Recent 

 Moll., II. 262 (as a subgenus under Planorbis), 



TropUHseus, Stein. (1855), in Schneck. et Mus. Her]., 7(1. 



Taphius, 11. aud A. Adams (1856), Genera Recent Moll., 11,264 (as a subgenus under Planorbis). — 

 Meek and Hayden (1865), Palseont. Upper Missouri, 107 (as a subgenus under Plan- 

 orbis). 



Mtnetus, H. and A. Adams (1856), Geuera Recent Moll., 11,262 (as a subgenus under Planorbis). — 

 Meek and Hayden i 1865), Palseont. Upper Missouri, 107 (as a subgenus). 



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

 Type. — Helix cornea, Linmeus. 



Shell apparently dextral ;* discoid or subdiscoid, the whorls being 

 nearly or quite coiled on the same plane ; right side generally flat, but some- 

 times either a little elevated or concave; left side more or less excavated ; 

 volutions rounded, compressed, or angular ; aperture crescentic or suboval, 

 sometimes dilated; peristome thin, incomplete, right margin projecting. 



The typical forms of this 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, however, it is made also 

 to include several subordinate groups, which depart more or less from the 

 typical species, though placed here by nearly all conchologists. 



The subordinate groups or sections of this genus adopted and proposed 

 by H. and A. Adams in their valuable work on the Genera of Recent Mollusca 

 are the following : 



1. planorbis, Miiller (typical). 



Shell with spire depressed, many-whorled, volutions generally 

 rounded ; aperture not dilated. — (Type as already stated.) 



2. helisoma. Swainson. 



Shell ventricose, concave on both sides ; volutions few, generally 

 angular on one or both sides; broadly rounded on the periphery. — 

 {P. bicarinatus, Say.) 



* Conchologists generally regard these depressed shells as being dextral; but O. A. L. Miirch 

 offers some reasons for viewing them as properly sinistral forms (Couch. Jour. (Paris), XI, 2d ser., 235). 

 This conclusion seems to be partly sustained by the form of the young of some American species, one 

 of which was described by Dekay as a truncated Physa. On the other band, however, monstrosities of 

 some foreign species with an elevated spire are generally dextral. 



