14 PLANORBIS TRIVOLVIS. 



side, tlie antepenultimate disappearing within 

 the umbilical cavity: aperture large, vaulted 

 anteriorly and slightly thickened within the 

 margin; its faces project considerably beyond 

 the planes of the shell. 



Color, rufous, or yellowish-brown. 



Geographical Distribution. Occurs in New 

 England, New York, Lake Erie, the Delaware 

 and Schuylkill, and in the Northwest Territory. 

 Dr. Richardson found it from Lake Superior to 

 Saskatchewan. 



OBSERVATIONS. 



The peritreme is not as oblique as in P. 

 glabratus, and both sides of the aperture pro- 

 ject beyond the penultimate whirl, except in the 

 variety ya//a^, pi. 3, fig. 1; whence it happens 

 in the latter, that the sides of the shell are 

 nearly parallel, from the slight increase of the 

 transverse diameter of the whirls. 



Bulla fluviatilis, Say, and Planorbis regularis, Lea, are 

 the young of this species, first named by the former, and 

 more recently revived by the latter. If it were a distinct 

 species, the original name should be preserved, as it is not 

 more objectionable than fontmalis, lacustris, ov fluviatilis, 

 applied to Physa and Ancylus; or fluviatilis, instead of its 

 proper name, to an Anodon. 



