166 UNIVALVE SHELLS 



-curred. It may be readily distinguished from P. 

 parvus, by its more convex form above, the spire 

 not being impressed, and by its very acute lateral 

 edge. It appears to be pretty closely allied to Plcu. 

 norbis nitidus of Europe, but it is larger, the umbili- 

 cus much more dilated, and the aperture does not 

 embrace the penultimate whorl so profoundly. 



4. P. * campanula tus Sinistral ; whorls longer 

 than wide ; aperture sub-campanulate. 



Inhabits Cayuga Lake. 



Cabinet of the Academy. 



Shell sinistral, not depressed : whorls four, slightly 

 striate across, longer than wide ; spire hardly con- 

 cave, often plane ; body whirl abruptly dilated near 

 the aperture, and not longer behind the dilatation 

 than the penultimate whirl ; suture indented well 

 defined to the tip, the summits of the volutions being 

 rounded; aperture dilated ; throat narrow abruptly; 

 umbilicus profound, the view extending by a mi- 

 nute foramen to the apex. 



Greatest length of the body whorl, 1-4 of an inch. 



Breadth from tip of the labrum, 1-2 inch ; at 

 right angles to the last, 2-5 inch. 



This shell abounds in some of the small streams, 

 which discharge into Cayuga lake, where it was 

 collected by Mr. Jessup, who presented specimens 

 to the Academy, and tome. It is readily distin- 

 guished from our other species, by the sudden dila- 



