LIMNylllB^EA. 23 



They are strictly aquatic in their habits, abounding in 

 the small quiet streams and stagnant ponds, feeding ex- 

 clusively on vegetable substances. They usually come to 

 the surface to breathe the free air, but their organs of respi- 

 ration must be adapted, in some species at least, to breathing 

 through the medium of water, as they are occasionally found 

 in circumstances precluding any possibility of an approach 

 to the surface. 



Their eggs are laid in clusters, surrounded by U gtilatineus 

 matter. 



Many of the species possess the power of gliding along 

 the surface of the water, shell downwards, and letting them- 

 selves down by means of a gelatinous thread. 



From the fact of my finding young individuals only in the 

 spring, and numerous dead full-grown shells during the late 

 autumn and winter, I presume they arrive at maturity in one 

 season. They are active during the spring, summer, and 

 autumn, but bury themselves in the mud during winter, at 

 least in the Northern States. 



The Limnaeidae have been grouped by some authors according 

 to the number of their horny jaws, but in the present stage of 

 knowledge of them it seems to me preferable to adopt that division 

 into subfamilies based upon the form of the shell, which is found 

 to be spiral and elongate, spiral and flattened, or non-spiral and 

 simply patelliform. 



The shells of some of the various genei'a present considerable 

 difference in form, but their characters are not as well marked or 

 reliable as in the Helicidse. I have therefore given, under the 

 genus, a description of the typical form, leaving to the subgenera 

 the descriptions of the various diverging forms. 



So variable are the species in each of the American genera, and 

 so imperfect is our knowledge of them, I have not attempted a 

 full description of each species at this time. It seems best to me 

 to give all the original descriptions both of true species and 

 synonyms (translated when not in English), and a fac-simile of 

 the original figure of each. My work must therefore be con- 

 sidered rather a repoi't on the present state of our knowledge of 

 the family than an exhaustive monograph. I am in hopes of ob- 

 taining material for a more perfect work at some future day. 



