304 THE CHICAGO ACADEMY OF SCIENCES. 



Shell: Small, depressed-conic, fragile, sides nearly parallel, 

 a trifle rounded but narrowed posteriorly; anterior slope long, 

 convex, with a decided "hump" toward the apex; posterior 

 slope short, concave; sides convex; apex prominent, elevated, 

 obtuse, directed posteriorly and a little to the right side; the 

 apex divides the shell into about three equal parts, one poste- 

 rior and two anterior; aperture as large as the shell, narrowed 

 posteriorly; peristome entire, simple, acute; color light horn; 

 interior of aperture whitish. 



Length, 3.00; width, 2.00; height, 1.25 mill. (12143.) 



2.75; " 1.50; " 1.00 " (12143.) 



" 5.00; " 2.75; " 2.00 " (13051.) 



Animal: With a rather short and wide foot, emarginate in 

 front, rounded behind; color plumbeous, finely dotted with 

 white, with a yellowish line extending longitudinally on the 

 head; head emarginate in front with lateral flaps; mantle 



FIG. 105. 



ANCYLUS RIVULARIS Say. (Original.) Animal viewed from the under 

 side (enlarged). 



simple, of good size ; tentacles short, tapering, eyes placed near 

 the inner bases of the tentacles; shell attached by a single 

 large muscle in the apex of the shell. The foot measures 3.2 

 mill, in length and F.OO mill, in width in an animal contained 

 in a shell 5.00 mill, in length by 2.75 mill, in width (Fig. 105). 



yaws: Thin, covered with papillae; the lateral jaws are 

 fused with the large upper jaw. 



Radula formula : ^3 +jf + \ ,+^f- +^5 ( 1 6 - I - 1 6) ; the 

 central tooth is simple as in.Limnaea; the lateral teeth have a 

 base of attachment longer than wide, the lower outer angle 

 expanded; the reflection is very broad and bicuspid, the in- 

 ner cusp being very large and semibifid while the outer cusp 

 is smaller; there are also two very small denticles on the edge 

 of the upper part of the outer cusp; the fifth lateral is some- 

 what narrower and the inner cusp is decidedly bifid, making 

 the tooth practically tricuspid; the marginal teeth are very 

 narrow, with a long, narrow reflection which is distally tri- 



