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 peers connected by a 
thin callus. 
Lene 1.00; width, 3.00; aperture ic hee 95; we, 1.30 mill. (10242.) 
As oe EE ‘a 1.10; 1.25 “ (10243.) 
4 1.25; oY tees s 3 Lee eee (10245.) 
‘3 1.50; “ 5.00; * rs £585 ae sore (12560.) 
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: 2°;+§+4+8+ 7% (18 — 1 — 18); central 
tooth with a base of attachment longer than wide, produced at 
Fie. 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 
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