254 POPULAR BRITISH CONCHOLOGY. 



masses, attached to water-plants. Although so much like 

 each other in general character and habits, the Limnmdce 

 differ extremely in the form of their shells. These are all 

 of a light horny substance, but present three distinct cha- 

 racters : PJi2/sa and Limnmis have spiral, oval shells, more 

 or less like Bidinius ; Planorhis has the spire compressed 

 so as to form a flat disc ; and Ancylus has a non-spiral, open 

 shell, like a limpet. 



LiMNiEus. — The shells are Hght, inflated, with a wide 

 aperture and small spire ; a twisted columella, and no oper- 

 culum. The animals have broad heads, with a pair of tri- 

 angular tentacles. 



The Limnmis is often found with the epidermis removed 

 near the spire and the shell corroded. Among the various 

 means of accounting for this, one has actually been resorted 

 to of accusing these animals of eating each other, commenc- 

 ing by nibbling through the spire ! It has also been ob- 

 served, that if the water-snails are accidentally left dry they 

 will assume the habit of land-snails, thicken the outer lip 

 of the shell, and make an epiphragm. Of the British spe- 

 cies, the shell of 



L. palustris is dark-coloured and rather oval, although 

 pointed ; that of 



