ZOOLOGY: W. J. CROZIER 
323 
close of the breeding season; (2) by the direct observation that a growth-line is 
formed during the winter period, and (3) by the coincidence of the findings 
based upon the counts of growth-lines with those derived from the modes in 
the frequency-distribution of sizes in the chiton population. 
The shell-plates are eroded at a characteristic rate, depending upon the 
particular environment, the erosion tending to obliterate the growth-lines. 
This difficulty in making the counts can, however, be overcome, and the meas- 
urement of the rate of the erosion itself affords an additional check upon the 
estimations. 
From the region of Great Sound 1090 chitons were examined; of these the 
ages of 881 (including 476 mature males and 352 females) were determined 
from the shell-markings. The growth-curve G4) in figure 1 was constructed 
to 
- Cms. 
9 
d 
7 
6 
5 
4 
5 
a 
_ / / 
1 
Years. 
I I I I I 
i i i i 
1 2 3 4 5 6 T d 9 JO /7 
FIG. 1 
Fig. 1. A, The modal lengths of Chiton tuberculatus at estimated ages one to twelve 
years; B, the growth of a chiton seven years old, as calculated from its shell markings. Great 
Sound, Bermuda. 
from the modes of the lengths in the respective age classes. These modes were 
closely coincident with the associated means. The data are heterogeneous 
to the extent that chitons from different situations within Great Sound have 
been lumped together. The mean growth-curve for either sex is sensibly the 
same. The form of the mean growth-curve — exhibiting an initial increase in 
growth-rate followed by the appearance of several overlapping 'cycles' of 
growth (Robertson) — is similar to that which may be calculated from the 
markings upon the shell of a single individual (fig. 1, B). 
The temporary depression in the rate of growth which commences with 
the third year is coincident with the onset of sexual maturity. This agrees 
with the similar correlation to be inferred from some brief statements by 
Heath 4 concerning the growth of several other genera of chitons. 
