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. 



Jaw : 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: -f-+f +|+i+|+|+! (16-1-16); cen- 

 tral tooth squarish, a little longer than wide, tricuspid, the cen- 



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 21-1-21 teeth. 



Distribution: Entire United States; British America. 



Geological distribution : Pleistocene; Loess. 



Habitat: Found plentifully under dead leaves, rotten logs, 



