260 PLANORBINiE. 



Sub-gen. isidora, Ehrenberg (Diastropha, Guildina). 



Shell ovate, umbilicated ; columella without any fold. 



albus, Turton. integer, Hold. 



diaphanus, Krauss. rivalis, Mat. and Rack. 



distortus, Hald. tropicus, Krauss. 



Guildingii, Swains. truncatus, Ferus. 



Sub-fam. PLANOKBINiE. 



Shell spiral, discoidal or depressed, rnany-whorled ; aper- 

 ture crescentic. 



Genus PLANORBIS, Guettard. 



Tentacles slender, filiform. Foot short, ovate. 



Shell dextral, discoidal ; spire depressed, whorls numerous, 

 visible on both sides; aperture crescentic, or transversely 

 oval; peristome thin, incomplete, the upper margin pro- 

 duced. 



Syti. Coretus, Adanson. Angarius, Martini. Orbis, 

 Schrbt., not Lea. Spirodiscus, Stein. Planorbina, Hald. 



Ex. P. corneus, Linnaeus, pi. 84, fig. 3. Shell, P. 

 corneus, fig. 3, a. 



The species of Planorbis are principally inhabitants of 

 temperate regions, and are very numerous in the northern 

 hemisphere ; they appear to be represented in the tropics 

 by the Ampullar tides, especially by the genus Marisa, the 

 shells of which bear a strong resemblance to those of 

 Planorbis. The tentacles of the animal are very long, and, 

 as might have been expected from the small size of the 



