ful reform in this science, separated it from that genus, but 

 did not distinguish it from his Bulimus ; thus uniting the 

 differently organized animals of land and water in one 

 i;roup. It is very true that some species of these two na- 

 tural genera resemble each other in the form of the shell, 

 but they may ahvays be distinguished by the fold of the 

 columella in the present genus. Lamarck, aw^are that the 

 nnimals were quite different in organization, and that the 

 one has tw^o tentacula and the other four, that one lives on- 

 ly in the water and the other altogether on land, placed 

 them in different families, and formed a separate genus 

 (as Muller and others had already done) under the above 

 name which is now almost universally adopted, for the 

 present aquatic group. The shell resembles Succinea, 

 which however is destitute of the fold of the columella, 

 and its animal has four tentacula. But of all the adopted 

 genera, it is most intimately related to Physa ; and Sower- 

 by in his "Genera," has reunited the two groups. The 

 peculiar fold of the columella exists in both, but the animal 

 of Physa has the mantle remarkably dilated, so as to ex- 

 tend over more or less of the exterior surface of the shell, 

 and is digitated on its margin, and the tentacula, which are 

 short and compressed in Limneus, are longer and almost 

 fdiform in the Physse. The shell also of the latter may be 

 distinguished by being heterostrophe. For these reasons 

 we agree wath Draparnaud and most modern Concholo- 

 gists in separating Physa as a genus. It would seem also 

 proper to separate Aplexa of Fleming, the animal of which 

 is destitute of a dilated mantle, though the shell itself re- 

 sembles that of Physa. Mixas of Leach may also, perhaps, 



be separated from Limneus ; it was formed for the L. 



PI. yj. 



