MARSH SNAILS. 203 
lamine ranged round a nucleus commonly placed 
in or near the centre. 
Unlike the great majority of the Comb-gilled 
Mollusca, the members of this family are confined 
to fresh waters, inhabiting lakes, streams, and 
marshes. ‘They are distributed all over the world; 
and occur in a fossil state as far back as the oolitic 
series. The fine globose shells which are found in 
the rivers of tropical countries, known as Apple- 
snails (Ampullaria), are but slightly separated from 
this family. 
GENUS BITHINIA. 
In addition to the family characters, those which 
distinguish this small group are, that the opercu- 
lum has a thick shelly coat on the inner surtace, 
and has the nucleus nearly central; and that the 
aperture of the shell has a slightly thickened rb, 
along the interior of the margin. 
Two species of this genus are found in the 
streams and ditches of this 
country, the more common of 
which is the Tentacled Bi- == 
thinia (Lithinia tentaculata). 
It is about half an inch in 
length; the shell is often 
covered with a blackish foul 
coat; the spire is composed of 
five whorls, the lowest of which 
: é : wy 
is swollen. The animal is = 
purplish black, with brillant BITHINIA TENTACULATA, 
yellow specks. eee genied). 
This genus, like the Mollusca generally, pro- 
duces eggs, while its fellow-genus Paludina is 
viviparous. The mode in which the eggs are laid 
