194 LAND AND: FRESH-WATER MOLLUSKS. 
is truncated or decollated from erosion or decay 
of the first whorls. 
It is a common and generally distributed 
species, in ponds, marshes, and lakes, usually 
about their margins. 
It differs from LD. stagnalis, in the shell being 
thicker and the whorls narrower. 
LimnzA TRUNCATULA—(the Small Mud Shell) 
(Pl. X., fig. 110).—The shell of this species 
differs from that of LD. palustris, in its minute 
size, not exceeding half-an-inch in length, and 
in the more rounded and deeply separated whorls, 
somewhat abruptly bent towards the suture. 
The above specific name, given to it by Miller, 
is derived from the truncated form of the whorls, 
and not from the decollation of the spire; for 
from its very habit it is not so lable to have 
the apex of its shell eroded. It received the 
name of fossaria from an English conchologist, 
Montagu, from its inhabiting ditches, and also 
that of minuta from Draparnaud, because of its 
diminutive size, it being the smallest of the 
genus. 
IL. truncatula is extremely variable in size, 
adult specimens being not unfrequently found 
one-third less in size than ordinarily; these 
inhabit high elevations and maritime marshes. 
It also presents very great differences in form, 
the degree of rugosity or smoothness, the pro- 
