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. spirorbis, 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 Planorbis 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 Mi- 

 chaud,t. 16. f. 12.) 



Q 4 



