GROWTH AND MORTALITY OP PACIFIC RAZOR CLAM 547 
Table 2. — Average width of Siliqua patula from California, Washington, and Alaska, and of Siliqua 
alia from Alaska, for each year of age 
Age, years 
S. patula 
s. 
alta 
Kukak 
Copalis 
Pismo 
100 W/L 
Number 
of speci- 
mens 
100 W/L 
Number 
of speci- 
mens 
100 W/L 
Number 
of speci- 
mens 
100 W/L 
Number 
of speci- 
mens 
23 
49.0 
1 
19 
43.0 
2 
18 
45.0 
2 
49.0 
2 
17 
44. 8 
3 
50. 0 
3 
16--_ 
45.8 
9 
49. 7 
9 
15 
44. 2 
33 
50. 0 
14 
14-. . 
43. 5 
21 
48. 8 
14 
13 
44.2 
29 
48.6 
17 
12 
43.4 
37 
50.0 
12 
11 
42.8 
29 
48. 5 
14 
10 
44.0 
27 
49. 2 
7 

42.0 
6 
42.0 
4 
48. 7 
3 
43.0 
6 
43.3 
12 


42.9 
19 
43.3 
10 
48.8 
5 
43. 2 
14 
44.3 
5 
48. 2 
3 
41. 2 
3 
41.0 
10 
i 41.8 
1 
46.0 
7 
- 
40.2 
39 
41. 7 
22 
i 41.3 
8 
'46.5 
28 

39.5 
123 
41.0 
39 
i 39. 4 
60 
1 43.9 
42 
38. 2 
13 
38. 6 
39 

.5. 
36. 0 
6 
37.9 
97 
48. 0 
24 
45. 8 
2 
37. 2 
11 
.5 
39.0 
6 
40.8 
117 
65.0 
4 
.2 
47. 5 
27 
.1 
87. 1 
6 
89.6 
3 
1 Ring measurement: For correct age subtract one-half year. 
In the foregoing comparisons of the form of Siliqua patula and S. alta we have 
used the average of each group. For the length of 11 and 12 centimeters sufficient 
material is available to show the variability. Here it is found that some specimens 
of S. patula were wider than some of S. alta. There is an overlapping between the 
two distributions which is found to amount to 6 specimens in 200, or 3 per cent of 
the entire number of clams of these lengths. There can be no doubt of a significant 
difference in relative width. 
POSITION OF UMBO 
By measuring the distance from the umbo to the posterior end of the shell, and 
expressing this as a per cent of the total length of the shell a numerical value for the 
location of the umbo is obtained. For S. patula from all parts of the coast this ratio 
is constant at any given length. During the period of rapid growth of the clam the 
umbo shifts toward the posterior end, indicating a relatively greater growth of the 
anterior end. After a length of 10 centimeters has been reached, the proportion of 
the length of shell on either side of the umbo is constant. (Table 3.) 
Comparable data are available for S. alta larger than 10 centimeters. In this 
species the umbo is more anterior than in S. patula. The anterior position of the 
umbo in S. alta, together with the narrower anterior end of the shell, gives it a dis- 
tinctive appearance by which it can easily be separated from S. patula. 
