4 8 4 



THE AMERICAN NATURALIST. [Vol. XXXVII. 



Table B. 



additional minor mode at 32 (frequency 14). 



fig. 2. cardium found the range of variation to be from 30 to 35, 

 forty-five specimens being counted. Dall remarks 

 qo millimeters that there is a slight tendency to fewer ribs in the 

 ong * southern than in the northern individuals of this 



species. It would be interesting to have a large number of 



specimens from different localities examined and plotted, to 



ascertain the exact amount of this variation. 



Cardium isocardia. 



Figures 3 and 4. 

 Table C. 



27 «8 «2_ 30 3]L S» 33 34 _35 J. 6 

 3 12 22 73 70 24 13 3 « 1 



Number of shells examined 222. The range of variation is 

 from 27 to 36. The mode is at 30 with a frequency of 33 and 



feature of the curve of isocardia is the polygonal instead of the 

 triangular form and the marked symmetry of the two sides. 

 A comparison of the curves obtained from an examination of 



