GROWTH AND MORTALITY OF PACIFIC RAZOR CLAM 
549 
in the larger sizes (12 to 16 centimeters). The maxima fall at widely different ages 
but at a common length of 5 to 8 centimeters. The highest absolute variability 
therefore corresponds in general to the period of most rapid growth but in all cases 
occurs somewhat after the inflection. 
If the relative variability is calculated, a different picture is obtained. The inter- 
decile range expressed as a per cent of the median * s vei T l ar g e in the younger 
stages, for example at a length of 0.35 centimeter it is over 200. From these high 
values it falls throughout the available life history until in large clams it is less than 
15 per cent of the length. 
Figure 3. — The variability (D) of each length of clams from five localities 
SEXUAL DIFFERENCES 
In the curves here presented the sexes have not been considered separately except 
in three localities. The shells of the two sexes are indistinguishable, and hence 
identification requires the examination of fresh material in which the eggs or sperm 
may be recognized by a hand lens. In order to see if sexual differences in size or weight 
were apparent, growth curves were constructed for three localities. At Swickshak 
152 males and 150 females between the lengths of 7.50 and 12.25 centimeters were 
measured. During the period of most rapid growth the differences were too slight 
and inconsistent to be of significance in spite of the fact that at this time such differ- 
ences are greatest. 
A series of 115 males and 113 females were available from Hallo Bay (Table 4); 
and since greater and more consistent differences were found in these than in other 
material, the two curves are reproduced in Figure 4. At the first winter the males 
average slightly the larger. During the period of most rapid growth, in this case from 
2 to 9 years, the males are the shortest and the curves then again cross, the two sexes 
having the same length during the tenth, eleventh, and twelfth years after which 
again the males are longer, though the differences are less marked than in the period 
of rapid growth. Incidentally the mortality of the females is higher as shown by 
the fact that the males outnumber them from 8 years on, and that from 16 to 19 years 
only males are represented, although the numbers are small. 
20751—31 2 
