CONCHOLOGIA CESTRICA. 73 



P. armigera, SAY. 



Planorbis armigerus, Say, Jour. Acad. Phila. F. S., 

 II., 1821. 



Shell, light brown, polished; whorls 4, F. 

 sub-cylindrical, and sub-carinate below ; um- p.' 147 

 bilicus wide, and deep; aperture rounded, 

 with six white teeth, far within the throat ; 

 the largest thin, oblique, running back from Aperture of 

 the left to the right side ; on the left of this fm ' 

 is a small one, and around the vault four 

 others; the teeth sometimes wanting. H. 

 2/4, W. 6)4, mill. 



Station, Schuylkill River, and tributaries. Chester 

 County. 



Genus ANCYLUS, GEOFFROY, 1767. 

 Shell, thin, patelliform, non-spiral ; apex di- Animal of 



Ancylus. 



rected to the right ; aperture wide ; pentreme [w. G. B.J 

 continuous, simple, entire. FigJ49. 



Animal, tentacles triangular; mantle in- 

 cluded ; pulmonary orifice protected by a 

 branchial appendage ; foot large. 



A. rivularis, SAY, Jour. Acad. Phila. F. S., II., 

 1817. 



Shell, delicate, depressed-conic ; apex ob- 



j i j [W. G. B.j 



tuse nearer to, and leaning towards, one Pi 15a 

 side, and one end ; aperture oval, narrower /z^ 

 at one end, entire; within milk white. H. |HI^~~^ 

 1/4, W. 3, mill. 



Station, on dead shells, and stones, in streams. Ches- 

 ter County ; frequent. 

 7 



