FAMILY 5. LYMNZANA. 115 
by succeeding writers on account of the peculiar formation of the shells 
which it is intended to distinguish. The growth of the Planorbes is 
indeed very characteristic: their shells are flat and perfectly discoidal, 
the whorls coiling over each other upon their own axis with such regu- 
larity, that it has been almost a matter of controversy with authors to 
determine whether the shell is dextral or sinistral. The writers of an 
earlier age were so much struck with this discoidal peculiarity of growth, 
that they were tempted to arrange the Planorbes with the Ammonites ; 
and whilst Linnzeus referred them to the Helices, Miiller laboured suc- 
cessfully to restore the genus that had been proposed by Guettard. Little 
was, however, known of the anatomy of these animals: Draparnaud, who 
devoted, himself almost exclusively to the examination of the land and 
freshwater mollusks, certainly gathered from their habits that they must 
be in some way allied to the Lymnee ; but this affinity even was not fully 
demonstrated until the appearance of an elaborate memoir on the subject 
by Cuvier. 
The shell of Planorbis may be described as being of a discoidal form, 
with the spire so depressed that the apex is scarcely prominent; the 
whorls are rounded, and conspicuous on both sides, the outer one being 
sometimes carinated ; the aperture is entire, and semilunar, with the mar- 
gins disjoined, and the lip is either simple or thickened, but never re- 
flected. 
Examples. 
Pl. CX@s Fig. 1. 
PLanorBis cornevs, Draparnaud, Hist. Moll. Fluv. et Terr., pl. 1. f. 42 
to44. Chemn., Conch., vol. ix. pl. 127. f.1116 to 1120. Encyclo- 
pédie Méthodique, pl. 460. f.1, a, b. 
Helix cornea, Linneus. 
Planorbis purpura, Miller. De Férussac. 
Pl. CXC. Fig. 2. 
PLanorris GuADALOUPENSIS, Sowerby, Genera of Shells, No.4. Chem- 
nitz, Conch., vol. ix. pl. 127. f.1118. 
Planorbis lugubris, Wagner. Deshayes. 
VOL. Il. Q 
