
398 THE AMERICAN NATURALIST. [VoL. XXXVI. 
Different limits for the classes were selected in the seriation 
so as to give in each case the most nearly normal distribution 
of frequencies. 
Laying off the frequency polygon by the method of rec- 
tangles gives Figs. 5 and 6, both curves of Type 4. The essen- 
tial data for these curves are: 
ANTERIOR, POSTERIOR, 
A .6783 6613 
Vi .0033 —.0037 
Us —.00000 138 —.00000297 
B, 00005 .O4I 
B, 74.47 1388.8 
E —142.94 -2771.47 
a (skewness) —.00028 -.022 
g (index of variability) 
.0287 +.00083 .0254 4.00073 
Discussion of results. 
I. The distance of the adductor muscle from the umbo, meas- 
ured along the line of migration, varies considerably zn its relation 
to the distance to the shell margin, measured along the same line 
produced. The extreme ratios are .591 and .754. The meas- 
49 mm. 
mm. 
‘urements in the latter case are To produce the former 
proportion we should have to have 37.3 mm. instead of 49 mm., 
— a change of 11.7 mm., or nearly x of the total measure- 
ments. 
2. While the range of variation then is comparatively large, 
yet the frequencies are well concentrated at or near the mode. 
In 93 per cent of the cases the position of the muscle would not 
vary 3 mm., in either direction from the mean position. 
3. Roughly speaking, we may say that the muscle is situated 
two-thirds of the way from the umbo to the margin. Accurately, 
the mean ratios are .6757 for the anterior muscle and .6608 
4. It was thought at the outset of the investigation that the 
results might indicate a difference in the rate of migration of 
the adductor muscles, one approaching the shell margin at a 
more rapid rate than the other. The negative skewness of both 


