

650 OUR FRESH-WATER SHELLS. 
place for certain species. With the results of a day's col- 
lecting before him he will probably tind the following genera 
of kieUrodthing water snails represented. 
Lymnea, in which the aperture of the shell is on the 
right side, when the shell is held with the apex upward, and 
the aperture facing you, the tentacles being broad and tri- 
angular. 
Physa, in which the aperture is always on the left side, 
if the shell is held in the position just described, and the 
tentacles slender. 
Planorbis, in which the shell is always coiled in a plane 
so that there is no elevated spire. 
Ancylus, in which the shell differs very widely from all 
the rest in having no twisted spire, but in having the shape 
of an oval flattened bowl. 
There is a common marine shell called limpet that the 
shell of Ancylus greatly resembles in form, though the ani- 
mals are entirely unlike. 
UNIES 
Physa. Ancylus 

Planorbis. 
We here give figures of these four characteristic forms, 
side by side, and these are represented in North America by 
about one hundred and twenty species, according to Binney. 
In the plate we have figured some of the prominent spe- 
cies of Lymneea and Physa, the cuts having been loaned us 
by the Smithsonian Institution, whose publications in this 
department have been of great value to the student in quest 
_ of these animals. 
In the explanation of plate the distribution of the species 
represented is given. We may add that the species of 
these genera are very perplexing to define, and will require 

