GROWTH AND MORTALITY OF PACIFIC RAZOR CLAM 
545 
from the umbo to the posterior end of the shell were measured. In a smaller number 
of cases the ventricosity or transverse diameter, the length of the ligament bed, and 
other dimensions were determined but these proved less useful. 
Although the form of animals, as, for example, the head length and size of eye in 
fish, have been used in systematic work, too little attention has been paid to the 
variability and the changes of form with age. In recent years Huxley and some of 
his students (1924, 1927, 1927a) have made a series of notable studies of animal form. 
They have found that in most cases the relation between part and whole may be 
represented by the formula 
y = bx k , 
where y is the length (or weight) of the part, and x that of the whole. 
Figure 2. — Average percental width of S. patula from California, Washington, and Alaska, S. alta from 
Alaska. A, For each centimeter of length; and B, for each year of age 
If a constant relation of this type persists throughout life we may distinguish 
two cases. In one the exponent k is unity and the formula becomes 
V = bx; 
that is, the part bears a constant relation to the whole, and the form does not change 
with size. This, Huxley calls “isogonic” growth. On the other hand, k may be 
greater or less than unitj^, indicating that the part is increasing or decreasing in 
relation to the whole, a type of growth called by Huxley “heterogonic.” Of course, 
neither condition may exist through life, but the differentia] growth ratio may change. 
WIDTH 
In order to present the length-width relation in the clam, we have calculated the 
percental width of the shells in a series of 1,330 individuals of S. patula and S. alta. 
These results are given in Tables 1 and 2 and presented graphically in Figure 2, 
