WATER. SNAILS. 191 
the shell is smaller, more slender, and tapering, 
and is the variety fragilis; though variable in 
its proportions, it always presents a very oblique 
aperture, exceeding in height half the length 
of the shell. 
The dental formula of this species is 4235= 
12,210; the central tooth is very minute; the 
laterals are large with two unequal prominent 
points, the outer the smallest. (See fig. 24.) 
Ha a a 
Fig. 24.— Portion of a transverse row of the lingual ribbon of 
L. stagnalis (Lovén). 
I. stagnalis prefers animal substances, and 
plays the part of “scavenger of the waters” ; 
its presence in aquariums is almost necessary. 
It has been observed devouring the larva of 
Dytiscus, a water-beetle. 
Fig. 25.—L. stagnalis. 
It is a sluggish animal, and may be seen on a 
