WATER SNAILS. 
205 
FRESH-WATER LIMPETS. 
Genus Ancylus ( 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- 
ncea , 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. 
Fig. 26 .—Portion of a transverse row of the lingual ribbon of 
A. fiuviatilis (Loven). 
In the subgeneric group, Yelletia, Gray , 
the animal and shell are dextral; the lingual 
dentition is different, the central part of each row 
being much arched and composed of a central 
