186 LAND AND FRESH-WATER MOLLUSKS. 
I. L. peregra , L. auricularia, and L. stagnalis, 
have the last whorl much enlarged. 
II. L. palustris , L . truncatula, and L. glaber, 
form another section, in which the spire is much 
elongated and the whorls gradually increase in 
size. 
III. L. glutinosa and L. involuta are separated 
by some authors from the true pond-snails, and 
placed in a sub-generic group under the name of 
Amphipeplea , because they possess a globular 
membranaceous shell, and the animal, though 
like Limncea , has the edges of the mantle, when 
the snail is in motion, extended so as to cover 
the shell. 
A Limncea is known in the Middle Purbecks, 
and the genus is represented by numerous spe¬ 
cies in the fresh-water beds of the Upper Eocenes, 
in the Isle of Wight; of the living species, 
L . palustris, L. peregra , and L . truncatula , first 
appeared during the deposition of the Mamma - 
liferous Crag at Bramerton and Bulcham; they, 
with L. stagnalis and L . auricularia , occur in the 
Pleistocene marls of Essex. 
Limn^a peregra— ( the Wandering Mud Snail) 
(PI. X., fig. 117).—This mollusk is the most 
widely dispersed and abundant, and, at the 
same time, the most variable of the fresh-water 
snails. 
The shell is ovate, thin, the colour varying 
