634 
BULLETIN OF THE BUREAU OF FISHERIES 
been used to determine the age of trees that in their structure show evidence of sea- 
sonal growth. Observers have established the presence of annual marks on the 
scales and otoliths of fish. Weymouth (1923) has shown in the Pismo clam and 
Weymouth, McMillin, and Holmes (1925) in the Pacific razor clam that there is a 
very definite relation existing between seasonal growth and the structure and external 
appearance of the shell. Rings are formed only once each year at the time of slow 
or suspended growth in the winter. Evidence of this cyclic growth in the Pismo clam 
has been presented by Weymouth (1923), and a retardation of growth during the 
winter months has been shown to occur in the razor clam by McMillin (1923), in the 
limpet ( Patella vulgata ) by Orton (1928), and in Tellina tenuis by Stephen (1929). 
By marking and holding Atlantic cockles ( Cardium edule), Orton (1927) affirmed the 
validity of the rings as measures, of age. 
AGE IN YEARS 
Figure 2. — Course of growth of Cardium corbis at Snug Harbor 
GROWTH 
In determining the size at the different ages the shells were measured radially 
from the umbo along the longest rib by means of calipers reading to tenths of milli- 
meters. This rib, in Cardium corbis, is located near the posterior end of the shell. 
The annual rings to which the measurements were made are very definite and readily 
noted in the cockles from northern waters. The determination of the annual rings is 
more difficult on those from southern beaches, where winter growth is less retarded. 
From measurements made in this manner the secular trend of the growth has 
been determined. The growth may be graphically represented by two methods, direct 
and logarithmic, and each type of curve has its particular value in the presentation 
of data. Figure 2 is an example of the direct presentation of growth and the curve is 
of the type most commonly used. This type of curve is constructed from the median 
length at each age, and is convenient for comparing the growth of closely related 
forms. (Brody, 1927.) 
The life of this clam is sufficiently long to permit satisfactory trends of growth to 
be determined and to eliminate the effect of chance succession of favorable or un- 
