OP CONCnOLOGT. 



ON AN ABNORMAL SPECIMEN OP PLANORBIS 

 BICARINATUS. 



BY GEORGE W. TRYON, JR. 



Plate 1, Fig. 4. 



The shell figured, is in the collection of Charles M. Wheat- 

 ley, Esq., of Phoenisville, Pa., and was collected by him at 

 Mexico, Owego Co., N. Y. It has assumed a turbinate and 

 scalariform shape, the spire being obtusely elevated, whorls 

 very convex, with deep suture, and the upper carina strongly 

 developed ; at the base it is narrowly umbilicate, the umbilical 

 region small, and bounded by the lower carina; the aperture 

 is small, ovate, somewhat expanded, and much thickened 

 within ; owing to compression, the two extremities of the lip 

 join upon the body, and are turned into the aperture, forming 

 a strong rib, which revolves within the shell. 



Numerous abnormal European Planorbes are illustrated in 

 Hartmann's magnificent work, "Erd und Slisswasser Gastero- 

 poden der Schweiz," but the species are all small ones. This 

 is the first American specimen which we have observed to de- 

 viate much from the normal form; reversed or scalariform 

 shells being very much rarer in America than in Europe. 

 Some of our species of Limnsea are sometimes slightly scalari- 

 form, and in the Viviparidse this is more frequent, but the de- 

 viation has never been observed to be very great. Melantho 

 Integra and decisa are both occasionally met with, reversed. I 

 believe that no scalariform Helix has been observed in the 

 United States, and the reversed specimens known do not ex- 

 ceed ten or twelve.* 



* See " Note on Variation in Species of Helix in Eastern North America," 

 in " Remarks on Certain Species of- North American Helicidae," by Thos. 

 Bland.- -Annals of the Lyceum of Nat. Hist., New York, VII , 1863. 



