LIMN^ADiE. 257 



The smaller species are liable to many distortions. 

 Mr. Sheppard describes a specimen of P. marginatum, 

 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 pre- 

 ceding whorls, which gives the idea of a serpent 

 coiled up. (Linn. Trans, xvi. 157.) We have in the 

 British Museum several specimens equally dis- 

 torted. 



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 conclusion. 



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 struc- 

 ture, as the whorls gradually glide from left to right, 

 down the imaginary axis. (See Michaud, t, 16. 

 f. 12.) 



3. The animal has its three orifices on the left 

 side of the neck, a character which distinguishes it 

 from all the other dextral Limnaeadce. But this is 

 only a displacement of the extremities of these 

 organs, for the organs themselves are placed on the 

 right side of the body, as in all the other dextral 

 JLimnccadce. 



