300 THE CHICAGO ACADEMY OF SCIENCES. 



nations the writer failed to note any differences of importance in 

 the radulae of these small species. The number of teeth seemed 

 to be always 17-1-17 or 18-1-18. 



Genitalia: Not examined. 



Distribution: Maine west to Nebraska, Great Slave Lake 

 south to Maryland. 



Geological distribution: Pleistocene; Loess. 



Habitat: Same as 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 SEQMENTINA, 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. 



Jaw: Narrow, arched, pointed. For Kadula, see under 

 armigera. 



Distribution: Europe, Asia, Australia, North America. 



SUBGENUS PLANORBULA Haldeman, 1844. 



All of the teeth in the aperture, except the last row, ab- 

 sorbed in the aduk. 



121. Segmentina armigera Say, pi. 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 



