No. 407.] PECTEN IRRADIANS LAMARCK. 869 
2. The prevailing number of rays in the right valves from 
all localities is seventeen ; in the left valves from Cold Spring 
Harbor it is seventeen also; but at the east end and the south 
shore of Long Island it is sixteen. Comparison of the aver- 
ages shows that the Cold Spring Harbor shells tend to have a 
comparatively large number of rays on both valves— in the 
mean 0.8 of a valve more than in the other localities. In the 
average number of grooves Cold Spring Harbor stands widely 
separated from the other two localities, which are closely 
related. 
3. Using as an index of variability the standard deviation c, 
it appears that the Cold Spring Harbor shells are possibly more 
variable than those from Cutchogue. However, the difference 
is less than the probable error, and no stress is to be laid on the 
fact. The same is true of the apparently greater variation of 
the South Shore shells. So we may conclude that, despite dif- 
ferences in the mean, the variability of the grooves is constant. 
This result accords with certain others obtained by counting 
integral variates. Duncker ('99, p. 328) says: “ While the 
average values of a character may differ widely in different 
form-units of the same species, the indices of variability remain 
fairly constant, not only in the form-units of the same species, 
but also in those of species belonging to different genera, even 
to different families. This fact does not seem to me to have 
been sufficiently regarded hitherto; the explanation of it is, I 
suppose, the constancy of the physiological capacity of a given 
organ for reacting to the individual causes of variation 
with respect to a given character. Some authors, bdwever, 
seem to assume a more or less constant relation between the 
height of the average and that of the index of variability of a 
character." I will not here discuss, as I propose to do else- 
where, the relation between the mean and the index of varia- 
tion. The matter needs special investigation. 
4. The variability of the right or lower valve is in every case 
less than that of the left or upper valve, and this difference 
in the case of the Cold Spring Harbor specimens is greater 
than the probable error. From this fact we may conclude that 
the right valve is the more conservative, or responds less to 
