324 
ZOOLOGY: W. J. CROZIER 
It is of interest to determine the duration of life of chiton in different locali- 
ties. In figure 2 are plotted the percentage frequency of the ages observed for 
chitons of both sexes. Chitons less than four years old are difficult to obtain 
in large quantities, because they live in crevices which are in many cases 
practically inaccessible. Therefore it is legitimate to consider only those indi- 
viduals which are estimated to be more than five years old. Beyond this age, 
C. tuberculatus tends more and more strongly to come out into exposed situa- 
tions. Therefore the relative proportions of the age-frequencies observed 
beyond the six to seven year-class are probably very nearly correct. Defin- 
ing the mean death point' as the age at which there are already dead 50% of 
loo 
90 
do 
7o 
60 
50 
40 
5o 
eo 
jo 
6 7 8 3 10 11 1Z 13 
FIG. 2. THE PERCENTAGE SURVIVAL OF CHITONS AFTER THEIR SIXTH YEAR 
Based upon records of 516 individuals six years old or over according to the indications of 
the shell markings. The 'mean death-point' is defined as that age at which 50% still sur- 
vive ; for this locality it falls in the eighth year. 
those chitons found to attain the age at which dying-off begins, it is seen (fig. 
2) that the natural termination of life comes on the average during the eighth 
year. This agrees, in general, with the estimated average age of shells found 
naturally dead. 
The growth-curve established for the region of Great Sound, Bermuda, may 
be used as a norm with which to compare the growth of Chiton tuberculatus in 
other localities. These observations might with advantage be extended to 
more southerly portions of the West Indian faunal region, where Chiton tuber- 
culatus is also found. 
Summary. — A curve of the growth of Chiton tuberculatus has been ob- 
tained for the population in one locality at Bermuda, on the assumption that 
the age of a chiton may be estimated from the growth-lines upon its shell. 
