54 LAND AND FRESH-WATER MOLLUSKS. 
and Valvata. Paludina is_ ovoviviparous, 
Bithinia and Valvata are oviparous; the oper- 
-culum of Paludina and Valvata is horny, of 
Bithinia shelly. Valvata is distinguished from 
the others by the presence of a long plume-like 
gill, which is partially protruded when the animal 
is crawling, and by the more circular whorls 
composing the shell. 
The lingual ribbon of Paludimde is very 
simple, and presents the same general features 
throughout the family, and places it in close 
proximity to the periwinkles and others com- 
prised in the family Littorinide. 
The general formula is 3.1.3; the number of 
transverse rows is very limited. 
Genus PALUDINA. 
PALUDINA VIVIPARA—(the Common Marsh Shell) 
(Pl. IV., fig. 26).—The specific name vivipara 
was given to this species by Linnzus, from the 
animal being ovoviviparous, that is to say, the 
egos are retained within the interior of the 
parent until they are hatched; it is a peculiarity 
possessed by other species of the genus. As 
the next species very closely resembles the. 
present one, it will be necessary to give a full 
description of it. 
The shell is conically oval, rather solid, of 
rather dull yellowish-green, with three broad 
