THE NATURAL HISTORY SURVEY. 305 
cuspid, the central cusp being the larger, and the outer edge 
of the reflection serrated by about five small denticles. The 
bifurcation of the inner cusp of the lateral teeth is difficult to 
see, but the writer is certain of the fact, as it has been dis- 
tinctly seen on a number of teeth. The cutting points are very 
well developed but are also difficult to see clearly. go rows 
were counted, and in one individual 20-1-20 teeth (Fig. 106). 
Genitalia: Not examined. 
Distribution: Northern, Middle and Western United States 
and Southern Canada south to New Mexico. 
Geological distribution: Pleistocene. 
Habitat: Found on living and in dead shells and on 
stones in small ponds and streams. 
Fie. 106 
Radula of ANCYLUS RIVULARIS Say. (Original.) c, central tooth; 
1, first lateral; 5, fifth lateral; 9, fourth marginal; 13, eighth marginal. 
Remarks: This small but distinct species may be known 
by its almost straight lateral outline and its apex, which is di- 
rected posteriorly and to the right side. It is quite abundant, 
but is almost always overlooked owing to its peculiar shape and 
inconspicuous habitat. The animal is very slow in movement 
and progresses similar to Planorbis,; it is able to turn its body 
half way around without moving its shell. The buccal organs 
can be plainly seen while the animal is feeding. This species 
was noticed using its lingual membrane with a “lapping” mo- 
tion, as described tor some of the Planorbes. Large specimens 
have recently been found by Mr. F. M. Woodruff, in the 
sloughs back of Miller’s, Ind. 
123. Ancylus tardus Say, pl. xxx, fig. 30. 
Ancylus tardus Say, New. Harm. Dissem., Jan. 15, 1830; Mrs. Say’s 
reprint, p. 26, 
Shell; Small, conic, fragile, sides rounded; anterior a 
trifle longer than posterior, convex; posterior slope straight 
or a trifle concave; sides rectilinear; apex prominent, elevated, 
obtuse, rounded, alittle posterior of the center but not directed 
laterally; the apex very nearly divides the shell into two equal 
