PHYSAD.E. 13 



and but one Paludina, has been yet observed 

 there. They prefer quiet cold waters, species 

 of Physa being sometimes found in springs 

 having a temperature nearly as low as ten de- 

 grees centigrade, and they may be sometimes 

 seen moving along the bottom, when the sur- 

 face is covered with ice. This fact tends to 

 prove that they can remain a long time beneath 

 the surface without requiring a fresh supply of 

 air; and it is not improbable that they can 

 remain deprived of access to the atmosphere, 

 for several days, weeks, or even months, when 

 the full action of their system is retarded by 

 cold. I have observed them moving about in 

 water at the temperature of three or four de- 

 grees centigrade, when species of Melania and 

 Paludina were torpid; the latter, in fact, are 

 just able to move, in the most languid manner, 

 in a temperature of eight or nine degrees. 



Like the Helicina?, this family is hermaphro- 

 dite, both sexes being united in the same in- 

 dividual. All the genera are oviparous, the 

 eggs being generally deposited upon the under 

 surface of stories, or upon sticks and leaves in 

 the water, many being enclosed in a voluminous 

 transparent glairy mass. This mass, in Pla- 



