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WATER SNAILS. 
Famity Limnaipm (Water Air-breathing Snails). 
THs very natural family comprises the fresh- 
water pulmoniferous snails; they all have short 
dilated muzzles, and two tentacles, with the eyes 
sessile at their inner bases. Nearly all the pul- 
monifera have only one jaw,* implanted supe- 
riorly between the lips; but species of the 
genera Limnea (excluding Amphipeplea) and 
Ancylus, and Planorbis corneus, have three jaws 
or, more properly, the labial armature is formed 
of three pieces: the one large (the true jaw) is 
placed transversely above the mouth, whilst the 
other two, less in size, are placed vertically one 
on the right and one on the left side. In the 
remaining species the mouth is armed with a 
single arched piece. The lingual dentition pre- 
sents widely distinct characters in the several 
genera. The stiff silky hairs, which are found 
on the skin of Neritinee, have been found by 
M. Claperéde to exist on most of our fresh-water 
* This organ 18 composed of hardened mucus, and con- 
tains a feeble quantity of carbonate of lime. 
