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.) 
a Og Us e COO" (12143.) 
PEs wee GS iN 2.00 “ (13051.) 
Animal: Witha 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 
Fie. 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 1.co mill. in width in an animal contained 
in a shell 5.00 mill. in length by 2.75 mill. in width (Fig. 105). 
Faws: Thin, covered with papillz; the lateral jaws are 
fused with the large upper jaw. 
Radula formula: Pra +37 +1 432 +35-5 (16—1—16); the 
central tooth is simple as in Limnza; 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- 
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