106 LIMNiEID^. 



and umbilicus are both depressed. Teutacles slender, filiform ; 

 foot short ; locomotion very slow. 



Planorbis, Guettard, 1156. 



Syn. — Coretus, Adanson, l*I5t. Orbis, Schrot. Spirodiscus, 

 Stein. 



Diatr. — 150 sp. "World-wide. Fossil, tO sp. Lias — . 



Shell discoidal, biconcave, the whorls visible on both sides ; 

 aperture small, rounded ; margin usually simple, sometimes 

 expanded. 



Animal with a short, round foot ; head short, with sessile eyes 

 at the inner bases of the slender tentacles. Lingual teeth sub- 

 quadrate, central bicuspid, laterals tricuspid, marginals serrated. 

 P. contortus^ a minute European species, has above 6000 teeth. 



The plan of the spiral in this genus is such as to yield readily 

 to pressure ; hence monstrosities are rather frequent. They con- 

 sist of a tilting up of the whorls on one side or even a conical 

 elevation of the spire. The smaller species, in America and 

 Europe, appear to be most liable to these distortions — P. con- 

 tortus particularly so. 



Type, P. corneus^ Linn, (ciii, 2*1). Europe. 



TAPHius, H. and A. Adams, 1855. Shell strongly excavated 

 around the umbilicus, whorls irregularly rounded ; aperture oval, 

 expanded ; columellar lip straight, oblique. P. Andicolus, d'Orb. 

 (ciii, 28). So. America. 



HELTSOMA, Swains., 1840. Shell ventricose, paucispiral, fre- 

 quently carinated around the spire; each whorl nearly enveloping 

 its predecessor. P. bicarinatus, Say (ciii, 29). United States. 



PLANORBELLA, Hald., 1842. Shell paucispiral ; aperture oblong, 

 somewhat irregular, the whorl swollen behind it. P. campanu- 

 latus, Say (ciii, 30). United States. 



ADULA, H. Adams. Shell with the whorls rounded and 

 numerous, deeply umbilicated on the upper, and convex on the 

 lower side ; aperture campanulate. PL muUivQlvis, Case. 

 Northern Michigan. 



MENETUS, H. and A. Adams, 1855. Shell depressed; whorls 

 rapidly enlarging, usually angulated ; aperture very oblique. 

 M.helo'icvs, d'Orb. (ciii, 31). So. America. 



GYRATJLUS, Agass., 1837. (Nautilina, Stein, H. and A. Adams, 

 1855. Girorbis, Agass., 1837.) Shell discoidal, whorls few, rap- 

 idly enlarging, periphery sometimes carinated; last whorl some- 

 times deflected. P. deJJectus, Say. 



BATHYOMPHALUS, Agass., 1837. (Spirorbis, Swains., 1840.) Shell 

 depressed ; whorls numerous, rounded, not carinated. P. 

 anaiinus, d'Orb. (ciii, 32). So. America. 



ANisus, Studer, 1820. (Trophidiscus, Stein.) Shell much 



