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: This interesting species is at once known by the 

 denticles within the aperture, which can only be seen with a 

 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 



FIG. 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 Limncea. Shell 

 patelliform. Hermaphrodite, but not capable of simultaneous 

 reciprocal impregnation, as in the Limnaeans." Dall. 



GENUS ANCYLUS Geoffrey, 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): With a very large foot, oval in shape; 

 tentacles short, stout, triangular, eyes situated at their inner 

 bases; mantle margin simple; pulmonary orifice protected by a 



