300 THE CHICAGO ACADEMY OF SCIENCES. 
nations the writer failed to note any differences of importance in 
the radule of these small species. The number of teeth seemed 
to be always 17-I-17 or 18-1-18. 
Genitalia: Not examined. 
Distribution: Maine west to Nebraska, Great Slave Lake 
south to Maryland. 
Geological distribution: Pleistocene; Loess. 
Habitat: Sameas parvus, but preferring sticks and stones, 
and frequenting deeper water. 
Remarks: This is the largest of our small Planorbies, dis- 
tinguished from P. parvus by its obtuse peripheral keel and de- 
flected aperture. Some distorted specimens show a scalariform 
tendency (No. 12120). Particularly large specimens have been 
collected at Bowmanville. 
Genus SEGMENTINA, Fleming. 
Shell: Dextral, discoidal, depressed; spire on a plane with 
all the whorls; the interior of the whorls with numerous trans- 
verse teeth; aperture circular or oval; peristome simple. 
Animal; Similar to that of Planorbis; foot narrow ante- 
riorly, but wider and larger behind. 
Faw: Narrow, arched, pointed. For Radula, see under 
armigera. 
Distribution: Europe, Asia, Australia, North America. 
SuspGENus PLANORBULA Haldeman, 1844. 
All of the teeth in the aperture, except the last row, ab- 
sorbed in the adult. 
121. Segmentina armigera Say, pl. xxx, fig. 32. 
Planorbis armigerus SAY, Journ, Phil. Acad., Vol. 11, p. 164, 1818. 
Shell: Dextral, flat, somewhat carinated above and be- 
low the periphery; color pearl-white to reddish-brown, some- 
times black; surface smooth, shining, lines of growth very fine, 
oblique; apex sunken below the level of the whorls, very small! 
and rounded; whorls four, regularly and slowly increasing, ob- 
tusely carinated above and below the rounded periphery; spire 
concave, exhibiting all the whorls; sutures impressed; base of 
shell rounded; umbilicus round, deep, rather wide, concave, 
showing nearly all the volutions; aperture subovate, a trifle 
oblique, armed with five teeth, one on the parietal wall long, 
thin, S-shaped, extending in an oblique direction from a point 
near the upper carination of the body-whorl to a point near 
