100 LIMN^ID^. 



surface of the water, anchoring or letting themselves clown 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 Limngeidae, and from 

 numerous observations and experiments, conies to the following- 

 conclusions : The Limnteidas, 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 facilit}^ of reaching the surface b}' 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 LimuiBidae 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 Limnaeidoe normally respire water, 

 and that they are compelled to come to the surface and respire 

 air only by unusually high temperature. (Mai. BL, xxiv, pp. 

 175, 176.) 



The following arrangement of the genera of the family is 

 mainly that proposed by Mr. Wm. H. Dall. 



Subfamily LIIIN^IN^. 

 Shell spiral, the spire more or less elongated. 



LiMN^A, Lam., 1798. 



Etym. — Limnaios^ marshy. Pond-snail. 



Distr. — 200 sp. Eui-ope, 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 



