
No. 425.] ANADONTA GRANDIS SAY. 399 
curves, however, would indicate a tendency toward smaller 
ratios ;. that is, a decrease in the relative distances from the umbo 
to the muscle impression. This may mean that the muscles 
migrate constantly less rapidly than the margins grow. Since the 
skewness is least in the anterior adductor curve, the anterior 
adductor tends to move toward the margin at a more rapid 
Fic. 6. 
Fic. 5. 


















885 .605 .615 .635 .655 .675 .695 .725 .745 163 -575 -595 615 -635 655 .675 695 -715 735 -755 ; 
1 8 37 489 Uny € 1.8 s y 95 «4 mw 4 t 
Fic. 5. — Frequency olye for anterior adductor ratios. 
Fic. 6. — Frequency polygons for pos:erior adductor ratios- 
rate than the posterior, although there is a tendency in both 
to lag behind the rate of growth of the shell margin. The 
greater variability of the anterior muscle position would 
indicate that it is the muscle most concerned in the evolu- 
tionary process. However, variability and skewness are both 
so slight that the above conclusions must be tentative until 
