PALUDIXA. 163 



small spiral shells,, with a discoid or rather elevated spire, 

 by round and convex whorls, a circular aperture, with con- 

 tinuous margin, and a simple and acute lip. The oper- 

 culum is acute and spiral, and the inhabitant uses it, 

 without doubt, as a door whereby to exclude the cold of 

 winter, during his long sleep. 



Concerning the second, Pahidina, we may briefly state 

 that shells of this genus are very numerous in the great 

 rivers of India and America, particularly near the sea, 

 and in places where the water is a little brackish. Their 

 shells are ovate or conoidal, covered with an olivaceous 

 epidermis ; the spire is somewhat turreted, the whorls are 

 round, and either smooth or carinated, and the aperture 

 is also round or ovate, the margin being continuous, and 

 the lip simple and acute. The operculum is horny and 

 orbicular, either waved or spiral. 



In the third genus, AinpuUaria, a peculiarity of structure 

 bears an obvious reference to existing circumstances. The 

 species mostly inhabit shallow stagnant pools, where, in 

 summer, the water is frequently dried up, and, in order to 

 meet the contingencies inseparable from such reverse posi- 

 tions, the animal is provided with two distinct apparatus, the 

 one for breathing in water, the other in air. They are thus 



M 2 



