100 LIMN AID A. 
surface of the water, anchoring or letting themselves down occa- 
sionally by means of a glutinous thread. As is frequently the 
-case in fluviatile shells, the apex of the spire is usually eroded. 
A. Pauly has studied the respiration of the Limnzide, and from 
numerous observations and experiments, comes to the following 
conclusions: The Limneide, under natural conditions, come 
to the surface of the water in order to breathe air at intervals 
varying from a few minutes to several hours, chiefly according 
to the facility of reaching the surface by creeping. Under water 
the pulmonary orifice is kept closed, and is not extended by 
water; only very young snails have it open and filled with water, 
and this only before they begin to breathe air. If bubbles of 
air are present, as in shallow ponds containing many water-plants, 
or in an aquarium, the Limneidze make use ‘of these bubbles for 
their respiration. Adult specimens kept from air can survive 
for ninety days, but they respire only by the skin, and never use 
the pulmonary sac as a water-respiring organ. But as the young 
snails, in the egg and some time after being hatched, receive 
water in their pulmonary orifice, it is possible that those which 
live at a considerable depth may retain this sort of respiration 
during their whole life, together with respiration by the skin. 
S. Clessin thinks that ‘the Limneide normally respire water, 
and that they are compelled to come to the surface and respire 
air only by unusually high temperature. (Mal. BL, xxiv, pp. 
175, 176.) 
The following arrangement of the genera of the family is 
mainly that proposed by Mr. Wm. H. Dall. 
SuBraAMity LIMNAINA. 
Shell spiral, the spire more or less elongated. 
Limnaa, Lam., 1798. 
Etym.—Limnaios, marshy. Pond-snail. 
Distr.—200 sp. Europe, Asia, America, north of the Equator, 
Polynesia. Fossil, 75 sp. Wealden—, Europe; Laramie—, N. 
America, 
Shell normally dextral, oval-oblong, thin, corneous, translucid ; 
spire sharp, more or less acuminated ; last whorl ventricose ; 
aperture oval, ample, rounded in front ; columellar lip with an 
oblique plait entering above. 
When the ponds are dried up in seasons of drought, these ani- 
mals bury themselves in the mud, strengthen the outer lip of 
their shells by an internal rib, and close the aperture by means 
of an epiphragm like hibernating Helices. Their mode of prop- 
agation is very singular—three or more individuals being united 
in a chain for that purpose. Leach has remarked that, in conse- 
quence of the sexual parts being distant from each other, one 
