480 HISTORY OF 



In the genus Paludina, the head is much smaller, and the 

 foot much larger, than in Melania. Two species are found in 

 some parts of the Susquehanna, the larger one, Paludina 

 decisa, having a short smooth light green shell, nearly an inch 

 long, whilst that of the other is smaller, rough with transverse 

 spiral lives, of a dull light green color, and with a rounder 

 aperture. 



Amnicola presents a shell which is a miniature representa- 

 tion of Paludina. Amnicola limosa is one eighth of an inch 

 long, and resembles Paludina decisa, but the aperture is pro- 

 portionally wider. Amnicola lustrica is of the same size, but 

 is more nearly globular, the aperture is circular, and the base 

 of the shell presents an opening. Amnicola granum resem- 

 bles the latter, and is our smallest freshwater shell, being less 

 than the one twentieth of an inch in length. 



Valvata tricarinata is our only representative of this ge- 

 nus, which is distinguished by the circular aperture and open 

 base. 



The members of the family of freshwater univalve shells 

 called Phisadae, though they live in waters of ponds and small 

 streams, breathe free air, and are therefore compelled to come 

 to the surface to breathe, which is effected by opening a 

 small aperture to admit the air. The shells are thin and deli- 

 cate, and of uniform tints. Physa heterostropha is extensively 

 distributed over the United States, and is our only species in 

 this genus, which has the peculiarity of having the turns of the 

 shell reversed, or turned in a contrary direction from most 

 spiral shells. Nearly allied is the genus Limnea, of which we 

 have the following species; L. palustris ; shell brown, oblong 

 conic, with six whirls, the surface frequently marked with 

 irregular elevated lines — length about an inch. It is a Europe- 

 an species, but those of this country were named L. elodes, by 

 Say, under the impression of their being a distinct species. 



L. desidiosa ; a light yellowish delicate shell, growing to the 

 length of 3-4 of an inch, and presenting numerous varieties. 



L. caperata ; very dark brown, approaching to blackish, 

 shell covered with line spiral elevated lines, 3-4 of an inch 

 long, 



L. humilis ; shell ovate-conic, short and inflated, less than 

 half an inch long, A slender variety, considered a distinct 

 species by some authors; has been named L. medicella. 



