298 THE CHICAGO ACADEMY OF SCIENCES. 



ovate, expanded, very nearly in the same plane as the last 

 whorl; peristome acute, thin, the superior margin produced 

 very much over the inferior margin, and expanded near the 

 junction with the body whorl; interior of aperture yellowish- 

 white or whitish; terminations of peristome connected by a 

 thin callus. 



Length, 1.00; width, 3.00; aperture length, .95; width, 1.30 mill. (10242.) 

 1.25; " 4.10; " " 1.10; " 1.25 " (10243.) 



1.25; " 4.00; " " 1.10; " 1.10 " (10245.) 



1.50; " 5.00; " " 1.53; " 1.50 " (12360.) 



Animal: Dark brown, lighter below; toot short, rounded; 

 tentacles filiform, whitish, with a dark line on the center of the 

 dorsal surface; respiratory groove long and narrow, whitish. 

 Jaw: With lateral plates present. 



Radula formula : ^-V+f +i+t +^-*V ( 1 8 I 1 8 ) ; central 

 tooth with a base of attachment longer than wide, produced at 



FIG. 100. 



Radula of PLANORBIS PARVUS Say. (Original.) c, central tooth; 

 1, first lateral; 9, first marginal; 10, 13, marginals. 



the lower outer corner and excavated in the center of the lower 

 margin; reflection bicuspid, the cusps short and wide; lateral 

 teeth with a subquadrate base of attachment; reflection wide, 

 tricuspid, the center cusp long and rather wide, the side cusps 

 shorter; marginal teeth modified laterals in being low and very 

 wide, the outer cusp splitting into 2-3-4 small cusps (Fig. 100.) 



Genitalia: Not examined. 



Distribution: Whole of eastern North America, west to 

 Manitoba and California. Dredged in Lake Superior, at a 

 depth of eight to thirteen fathoms. 



Geological distribution : Pleistocene; Loess. 



Habitat: Found in great abundance on the stems of water 

 plants, submerged objects and among floating vegetation, such 

 as Spirogyra. 



Remarks: This is our most common small Planorbis, dis- 

 tinguished by its rounded aperture and periphery. It is so 



