LIMX&ADJE. 231 



: Tentacles short, compressed, triangular, without any 

 auricle at the base. (Limnaeana.) 



1. LIMN^US. (Mud Shell.) 



Animal with a short broad foot, broad short com- 

 pressed tentacles, without any auricles at the base, 

 a large central spiral body, and a simple-edged 

 mantle, covered by an external ovate, thin, dextral, 

 transparent spiral shell with an ovate mouth, having 

 a single oblique plait on the middle of the column 

 running into the axis. (See p. 219. f. 6, 7.) 



This genus is known from Amphipeplea and Physa 

 by the edge of the mantle not being produced so as 

 to cover the shell, and by the inner lip not being 

 extended over the body whorl of the shell. It is 

 known from Aplexus by the shell being dextral, and 

 having the pillar-plait ; the latter character, and 

 their greater solidity, will distinguish even the re- 

 versed monstrosities of these shells from that genus. 



The apex of the shell is often eroded or truncated ; 

 that is to say, as the upper part of the body is with- 

 drawn from the tip, and the body moves forwards into 

 the larger part of the shell, it forms a septum behind, 

 and the part that is thus separated even- 

 tually falls off. Mr. Jeffreys, not being 

 conversant with the physiology of the 

 Mollusca, gives a curious explanation 

 of this phenomenon, which is not un- 

 common in the terrestrial and marine, as well as 

 fresh-water Mollusca. " In the absence of other 

 nourishment, they (the Limncei) will even devour each 

 other, piercing the shell near its apex, and eating 

 away the upper folds of the inhabitants. This ac- 



