APPENDIX. 261 



PLANORBIS. 



t^*^ 1. P. deflectus. Shell dextral, depressed ; whorls nearly 

 five, minutely and regularly wrinkled across, wider than 

 long, with a much depressed rotundity above, descend- 

 ing to an acute lateral edge below the middle ; spire not 

 impressed ; suture indented, but not profoundly ; beneath 

 a little concave in the middle, exhibiting one-half of each 

 volution to the apex ; whorls flattened, slightly rounded ; 

 aperture declining very much, suboval, the superior por- 

 tion of the labrum considerably surpassing the inferior 

 portion and taking its origin a little above the carina ; in- 

 ferior portion of the labrum terminating on the middle of 

 the inferior surface of the penultimate whorl. 



Greatest breadth two-fifths of an inch. 



This shell was presented to me by Dr. Bigsby, who col- 

 lected many specimens in the waters of the North-west 

 Territory. 



It resembles the exacuous nob. but the aperture does 

 not embrace so large a proportion of the preceding volu- 

 tion, and the volutions on the inferior portion of the shell 

 are consequently more obvious, and the umbilicus is but 

 slightly indented ; the upper portion of the labrum does 

 not extend so far beyond the lower portion, the aperture 

 declines much more, and the carina is less acute. It has 

 also an affinity for the carinatus of Europe, but in addi- 

 tion to other differences the aperture of that species de- 

 clines but little, if at all, and the carina is an elevated re- 

 volving line. The aperture embraces the penultimate vo- 

 lution about as much as in the rotundatus of Europe, to 

 which our shell is also allied, but difiers in its declining 

 aperture, and the less degree of rotundity of its whorls on 

 their upper surface. PI. 15, fig. 8. 



Vol. II. 34 



