188 THE CHICAGO ACADEMY OF SCIENCES. 
pulsations per minute were counted when the animal was con- 
tracted, and one hundred and forty-eight to one hundred and 
sixty when it was extended. 
Yaw: Long,narrow,bow-shaped, the ends bluntly pointed; 
upper margin with a median depression, lower margin with a 
very wide, blunt median projection (Fig. 40, J). 
Radula formula: 3+8+8+1+5+4+8+32 (16—1I—16); cen- 
tral tooth squarish, a little longer than wide, tricuspid, the cen- 
J 
Fic. 40 
Radula of ZONITOIDES ARBOREUS Say. (Original.) c, central tooth; 
1, first lateral; 6, first marginal; 10, fifth marginal; J, jaw. 
tral cusp very long and wide and reaching below the base of 
attachment, lateral cusps short; lateral teeth five in number, 
similar to,central, but bicuspid, the inner cusp very long, the 
outer cusp short; marginal teeth eleven in number, thorn- 
shaped, all but the first three (which are bicuspid, the outer 
cusp being very small) unicuspid; the teeth decrease in size as 
the side of the membrane is approached (Fig. 40). . There are 
about 80 rows of teeth. Morse gives 21-12! teeth. 
Distribution: Entire United States; British America. 
Geological distribution: Pleistocene; Loess. 
Hatitat: Found plentifully under dead leaves, rotten logs, 
