WATER SNAILS. 205 
FRESH-WATER LIMPETS. 
Genus Ancytus (fresh-water Limpets). 
THIS generic group is at once recognised by 
the conical, limpet-like, thin shells; the apex is 
posterior and sinistral. The animal is like Lim- 
neea, but is more allied to Planorbis; its tentacles 
are cylindrical and not triangular. The mouth 
is armed with a jaw, composed of three rudi- 
mentary pieces. ‘The lingual dentition presents 
the following characters: the central tooth is 
minute, the lateral teeth, thirty-seven in number, 
have long recurved hooks, and are at first simple, 
but becoming ultimately narrowed and minutely 
toothed ; there are 120 transverse rows. 
LLLPFUW 
Fig. 26.—Portion of a transverse row of the lingual ribbon of 
A. fluviatilis (Lovén). 
In the subgeneric group, VELLETIA, Gray, 
the animal and shell are dextral; the hngual 
dentition is different, the central part of each row 
being much arched and composed of a central 
1 ae 
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Ht 
