302 THE CHICAGO ACADEMY OF SCIENCES. 
Distribution: Eastern, Middle and Western States and 
Canada. 
Geological distribution: Pleistocene; Loess 
Habitat: Found plentifully in the smaller streams, crawl- 
ing over submerged sticks, stones and water plants. 
Remarks: Thisinteresting species is at once known by the 
denticles within the aperture, which can only be seen witha 
good lens. It is quite a common species, under favorable cir- 
cumstances congregating by hundreds. Its movements are 
rather rapid, and when in progression its tentacles are con- 
stantly moving about. When eating, the mouth opens and 
41 45 
Fie. 102. 
Radula of SEGMENTINA ARMIGERA Say. (Original.) c, central tooth; 
1, first lateral; 10, first marginal; 11, second marginal; 15, outer marginal. 
closes very rapidly. It is found in the three regions of the 
area. 
FAMILY ANCYLID. 
“Lateral jaws present. Teeth resembling Zzmn@a. Shell 
patelliform. Hermaphrodite, but not capable of simultaneous 
reciprocal impregnation, as in the Limnzans.” Dall. 
Genus ANCYLUS Geoffroy, 1776. 
Shell: Patelliform, thin, conic; apex turned to the left, 
posterior; aperture as wide as the whole shell; peristome en- 
tire, simple; interior of shell with a subspiral muscle-scar. 
Animal (Fig. 103): Withavery large foot, oval in shape; 
tentacles short, stout, triangular, eyes situated at their inner 
bases; mantle margin simple; pulmonary orifice protected bya 
h, ioe 
