UNIVALVES. 
53 
are at first exceedingly unlike the parent; they 
are provided with a delicate nautilus-like shell, 
and closed by an operculum ; on each side of the 
head there extends a fin-like membrane, the 
edges of which are fringed with cilia, by means 
of which they swim within the contents of the 
egg. In most of the oviparous snails, on the 
rupture of the egg-cases, the young swim forth 
with great activity by the action of the lobes, 
and are dispersed far and wide. 
Habits , Foody &c.—Paludina is a sluggish 
animal; it feeds on vegetable matter, and pre¬ 
fers nearly stagnant waters, or very slow-running 
rivers with a bottom of soft mud; it buries 
itself for weeks in the mud, and crawls up at 
intervals. 
II.— Description oe the Species. 
The fresh-water Gasteropods of Great Britain 
are grouped in the families Paludinidoey Litto- 
rinidcBy and Neritinidce . 
Familv Paludinim] [Marsh Shells). 
This family contains a few genera of fresh-water 
snails distributed in all parts of the world. 
In Great Britain three generic groups occur : 
Paludina , the type of the family; Bithinia } 
