106 LIMN AIDA. 
and umbilicus are both depressed. Tentacles slender, filiform ; 
foot short ; locomotion very slow. 
Puanorsis, Guettard, 1756. 
Syn.—Coretus, Adanson, 1757. Orbis, Schrot. Spirodiscus, 
Stein. 
Distr.—150 sp. World-wide. Fossil, 70 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, x» 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, 27). Europe. 
TAPHIUS, H. and A. Adams, 1855. Shell strongly excavated 
around the umbilicus, whorls irregularly rounded ; aperture oval, 
expanded ; columellar lip straight, oblique. 2. Andicolus, d’Orb. 
(cill, 28). So. America. 
HELISOMA, 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. 
ApuLA, H. Adams. Shell with the whorls rounded and 
numerous, deeply umbilicated on the upper, and convex on the 
lower side; aperture campanulate. Pl. multivolvis, Case. 
Northern Michigan. 
MENETUS, H. and A. Adams, 1855. Shell depressed ; whorls 
rapidly enlarging, usually angulated; aperture very oblique. 
NM. heloicus, VOrb. (ciii, 31). So. America. 
GyRAULUS, 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. deflectus, Say. 
BATHYOMPHALUS, A gass., 1837. (Spirorbis, Swains., 1840.) Shell 
depressed; whorls numerous, rounded, not carinated. P. 
anatinus, d’Orb. (ciii, 32). So. America. 
ANISUS, Studer, 1820. (Trophidiscus, Stein.) Shell much 
