ZOOLOGY: W. J. CROZIER 
327 
influences is reflected in the fact that the chitons at Cross Bay die more quickly 
than do those in Great Sound (fig. 2). 
The death rate is not the same for the two sexes. According to the results 
embodied in figure 3, the female chitons, on the whole, tend to die off more 
rapidly than do the males. This results in a preponderance of males among 
the oldest members of the population. 
Independently of this effect, however, there are apparent in the present 
data some curious variations in the proportion of the sexes within the suc- 
cessive age-groups. The fact that these variations have been found in two 
distinct localities favors the view that one is not dealing with chance fluctua- 
tion. With one exception 3 the Placophora are, so far as is known, dioecious, 
and in Chiton|tuberculatus the sexes are readily distinguishable. To what 
FIG. 3. THE RELATIVE DEATH-RATES OF MALES AND FEMALES (FROM RECORDS OF 912 
INDIVIDUALS) 
extent the fluctuations exhibited in the sex-ratio are due to environmental 
influences must be decided by further observation. 
The sex of 1518 chitons was determined by dissection. The percentage of 
males (R<?) in this series was 53.3; 1009 chitons from the region of Great 
Sound gave a ratio = 54.9; 509 from Cross Bay, a ratio R^ = 50.4. 
With the data arranged to show the proportion of males at each age, in these 
localities, a progressive variation is evident (fig. 4). At first, there is a con- 
siderable preponderance of males, which, with fluctuations suggestively par- 
allel in the two cases, is gradually decreased, until the differential death-rate 
becomes effective. 
The chitons above seven years of age, which tend to occur in small groups 
scattered along the shore, exhibit a preponderance of males. Therefore it 
probably comes about, as a rule, that the eggs of a given female of this age or 
older stand a good chance of being fertilized, since solitary individuals of 
Percept. 
6 7 d 9 /o // 
