LIMNEA. 



The tongue of the Limneans is almost con- 

 tinually employed in the apparent office of 

 taking food, as may be readily observed when 

 the animal floats with the foot and mouth up- 

 wards. I have, on such occasions (in Physa 

 heterostropha) observed bits of matter to be 

 swallowed, and have found the water to be full 

 of Volvox globator, and other Protozoa, which 

 are probably brought within reach by the 

 ciliary currents. 



The following subgenera are indicated without any re- 

 ference to the question of their correctness. The number 

 happens to be five, as in Planorbis and Physa; but in the 

 lower classes, they probably much exceed this number. 

 The number five generally obtains, because, in genera, a 

 greater number of distinct forms would tend to run the 

 sections into each other; and not because Nature calls for 

 a particular number. Still, as strong arguments have been 

 brought forward in support of a quinary arrangement, I 

 think it but fair that naturalists should give it an impartial 

 and unprejudiced examination, in their peculiar depart- 

 ments. 



Subgenera of LIMNEA. 



LIMNEA, LAM. Shell lengthened, aperture slightly ex- 

 panded. 



L. appressa, pi. 5. 



RADIX, MONTFORT. Shell short, greatly expanded. 

 R. raacrostoma, pi. 11. 



