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. 90 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. 



FIG. 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, pi. 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, a little posterior of the center but not directed 

 laterally; the apex very nearly divides the shell into two equal 



