254 PNEUMONOBRANCHIATA. 



or in water among grass, while the larger one is found 

 in canals and nearly still rivers, which may account 

 for all the difference between them ; for we have often 

 been inclined to consider varieties as distinct, because 

 they were found in different situations, whereas the 

 difference of situation may be the only cause of the 

 variation ; which would probably disappear if they 

 were placed and allowed to breed in similar circum- 

 stances to the other variety. 



Mr. Jenyns informs me, that he has a British spe- 

 cimen, which he thought was the Physa alba of Tur- 

 ton (which is undoubtedly a Sicilian species), differing 

 from the general form of Physa fontinalis in the hinder 

 part of the last whorl being rather angular, as in 

 Limn&us stagnalis. 



Mr. James Sowerby has sent me a specimen of the 

 long-spired variety, under the name of Physa acuta ?, 

 which he received with Chara aspera from Anglesea, 

 in 1833, and which he has continued to breed in his 

 water-butt ; but he says it differs from the common 

 species in the form of the animal, which he thus de- 

 scribes : 

 " PHYSA (acuta ?). Animal with setaceous antennae ; 



an acute tail keeled upon the back ; a two-lobed 



mantle, of which one of the lobes covers the colu- 



mella, and is five-parted, the other is turned upon 



the spire, and is three-parted." 



The animal has the habit of throwing its shell 

 about in an extraordinary manner, either in defence 

 or to remove obstructions, continuing at the same 

 time fixed by its foot. Probably this motion is some- 

 times occasioned by a minute worm (the Gordius in- 

 quitinus of Muller) which infests this and many other 



