PLANORBIS. 
231 
sometimes called the under side the upper^ and vice 
versa. 
This genus is so named from the flattened and 
horizontal coil of the volutions^ by means of which 
the whole of the gyrations may be seen on each of 
the sides. 
The smaller species are liable to many distortions. 
Mr. Sheppard describes a specimen of P, marginatus 
with the volutions nearly disjointed or pulled out ; 
another of P, spirorhis^ in which the volutions ap- 
peared as if pressed out from the base towards the 
apex, and being almost disjointed, caused the shell to 
resemble a little basket ; another of P, vortex with 
the mouth enlarged and turned over the preceding 
whorls, which gives the idea of a serpent coiled up. 
{Linn, Trans, xvi. 157.) We have in the British 
Museum several specimens equally distorted. 
The Planorbes have been considered by many 
authors as sinistral shells. M. Desmoulins {^Act, 
Soc, Linn, Bord, iv. 273.) examined the question in 
detail, and came to the following conclusions : — 
1. That the shell of Planorhis is essentially dex- 
tral. 
2. The upper part of the shell is invariably indi- 
cated by the more advanced edge of the mouth, and 
not by the sinking in of the tip of the spire, which 
sometimes, does not exist. The monstrosities of 
these shells, which are not uncommon, also show 
this structure, as the whorls gradually glide from 
left to right down the imaginary axis. (See 
chaud^ t. 16. f. 12.) 
