GEOWTH AND AGE AT MATUEITY OF THE PACIFIC RAZOR CLAM 217 
The method is that which has long been unquestioningly applied to trees, which, 
in their structure, show evidences of the number of seasons through which they 
have passed. The annual nature of the rings " visible in the shell of the clam 
has been clearly proved by the senior author (Weymouth, 1923) in the case of 
the Pismo clam. The same method has already been applied to the razor clam 
(McMillin, 1923). In brief, it rests upon the fact that the clam does not grow 
at a uniform rate, growth being rapid in summer and slow or absent in winter. 
This is illustrated by the results obtained by following the young of the Pismo 
clam through two years (Weymouth, 1923) and the small razor clams tlirough 
one year. 
Figure 10 (reproduced from McMillin, 1923) shows the absence of growtli 
during the winter. The possible reasons for this, chief among which appears to 
be temperature, are discussed in detail in the papers referred to and need not be 
considered further here. For our purpose it is sufficient to note that the rings are 
formed during the season of slow growth. This is confirmed by many observa- 
tions on the razor clam, in that the relation of the rings to the growing margin 
of the shell in clams small enough to show appreciable annual growth is alwa^^s 
what might be expected for a winter formation of the ring. They lie close to the 
margin in the spring, farther away in the summer, and farthest in the fall. 
This is further supported by the study of size frequencies. During the winter 
all clams were taken from a certain area of beach and their total lengths recorded. 
By tabulating these in size classes a frequency table was obtained from which a 
curve was constructed (McMillin. 1923; fig. 8). This curve showed modes the lo- 
cation of which corresponded to the lengths of the annual rings for the second and 
third years. After the third year the overlapping in size of the different year 
classes obliterates the modes of each class. 
The annual nng is a structural feature of the shell and not a superficial mark. 
In the broken shell a layer marking the surface of the shell at the time the ring was 
formed can be seen extending from the ring on the present surface through the en- 
tire shell. A white line remains on the underside of the periostracum when it is 
peeled off. 1 Jiis is found to mark the location of the annual ring. Some feature of 
the growth about the time of the formation of the ring causes the periostracum to 
become fused with the calcified portion of the shell, thus giving emphasis to its 
location. From these facts it is clear that a ring of definite character is formed 
once and only once each year and is therefore a definite indication of age. 
The difficulties in applying this method are of three kinds. (1) Injury to the 
edge of the shell in summer may retard the growth and cause a check that might 
be confused with the annual ring. (2) A first and sometimes a second ring is 
difficult to determine on clams 12 or more years of age, as these early rings are 
formed on very thin shells and may be removed by erosion. (3) The crowding of 
the annual rings near the edge of the shell, due to the very small increment of 
growth, makes the location of the last rings in old shells indefinite. From the 
above causes some records may be in error, but the number and magnitude of these 
are not large, and because of the great number of observations the general results 
can not be appreciablj' affected. 
