274 THE CHICAGO ACADEMY OF SCIENCES. 



length of the entire shell; sutures well impressed; aperture 

 roundly-ovate, more or less expanded; peristome thin, acute, 

 sometimes expanded, in old specimens thickened by a heavy 

 deposit within; the peristome is white and there is a band of 

 very dark brown which edges the callus deposit; columella 

 oblique, reflected, with a large fold across the middle, and cov- 

 ered by a heavy, whitish, testaceous deposit which is more or 

 less spreading; umbilicus closed by the spreading callus and 

 reflected columella, but the region is indented and the umbili- 

 cus is sometimes narrowly open. 



Length, 27.50; width, 9.50; aperture length, 12.00; width, 5.00 mill. (9323.) 



" ' 23.00; " 9.00; " " 11.00; " 5.00 " (8114.) 



24.00; " 10.00; " " 11.50; " 5.50 " (9884.) 



30.00; " 12.00; " " 14.00; " 7.10 " (8115.) 



26.00; " 12.00; " " 12.25; " 7.00 " (8115.) 



20.00; " 9.00; " " 9.00; " 4.50 " (9695.) 



15.50; " 7.00; " " 8.50; " 3.50 " (9695.) 



26.50; " 11.00; " " 11.00; " 6.00 " (9695.) 



Animal: With a short, wide foot, rounded before and be- 

 hind; tentacles short, triangular; color black, lighter below, 

 the body spotted with white which shows through the shell. 

 Heart situated as usual, pulsations regular, eighty to eighty- 

 one per minute. Length of foot 8.00, width 3.00 mill. 



Jaw: As usual. 



.fa^/tf formula: ii+f+f+l+l+f+fi (34-l~34)l cen- 

 tral tooth as usual; lateral teeth of the usual type, bicuspid; 

 transition teeth at first like laterals but tricuspid, the central 

 cusp the largest (eleven) but soon (thirteen) the inner cusps 

 become more equal and the outer cusp small; marginal teeth 

 of the usual type (Fig. 89). In one membrane examined 

 (Fig. 90) the first lateral to the right of the central tooth had a 

 bifid outer cusp. This was observed in all the first laterals in 

 this membrane. 



Genitalia: Not observed. 



Distribution: North America, Europe, Asia; circumpolar. 

 Alaska (Randolph). 



Geological distribution : Pleistocene; Loess. 



Habitat: Found in small streams and rivers, ponds and 

 lakes, attached to floating sticks and submerged water plants. 



Remarks: This is a very common and also a very variable 

 species. It is always a wide, more or less fusiform species, 

 with the aperture and spire equal, or the latter a trifle longer, 



