Z PLANORBIS. 



SHELL an involute discoid spiral, with the 

 turns visible on both sides; aperture lunated, 

 modified by the penultimate whirl, and more or 

 less oblique, causing the shell to incline towards 

 the left: apex not confined to either side. 



f Apex upon the left side. P. bicarinatus. 



tf Apex upon the right. P. exacutus. 



fft Apex visible on both sides. P. deflectus. 



OBSERVATIONS. 



There has been some difference of opinion 

 as to whether this genus is generally dextral 

 or sinistral, and the reader is referred to the 

 work last cited (which includes Deshayes' more 

 important remarks) for views upon both sides 

 of the question; but as this is not yet settled to 

 the satisfaction of all, a few remarks will be 

 added to what has been already written. 



When the animal of P. bicarinatus or P. 

 corneus is placed so that the margin of the 

 aperture rests horizontally, (its normal position,) 

 the shell leans toward the left, because the right 

 edge of the labrum is produced farther than the 

 left,* which brings the spire to the lower, and 



* This view is taken by Deshayes, and is the correct 

 one; as any one can ascertain by examining the animal 

 when in motion. (PI. 1, fig. 2.) 



