GROWTH AND AGE AT MATUEITY OF THE PACIFIC RAZOR CLAM 203 
and closely placed. While the number and placing of the rings may distinguish 
shells from various localities, these, as we shall show, are the result of environ- 
ment and not specific differences. It is also found that clams on different sections 
of on© beach grow at different rates. Specimens from places only a few feet apart 
but separated by some obstruction, such as the Grays Harbor jetty, may show 
decidedly different shell markings, and the growth curve constructed from shell 
measurements reflects the distinction. 
The weight of shells from different beds also varies according to the rate of 
growth. As shown in Figure 13, the number of years necessary for clams to 
-47 
'-35 Im^MnUi^^^^Mn^i^^BB^HMjnHM^^nH^nBH^^^^H 
/ 3 S 7 9 // /3 IS 17 19 
I 1 1 I I I I I I I 
Affe in Years 
Fig. 1. — Showing increase in relative width with age. The solid line shows the median width at 
each age and the broken lines the 10th and 90th percentiles, between which are included 80 per 
cent of the widths. (Compilation of data derived from all sources.) 
reach a certain size varies with the beds. Each year a new layer is added to the 
inner surface of the shell. In clams that have not reached their full size a thick- 
ened portion of the new layer extends beyond the margin, thereby increasing the 
size. The additions on some old clams increase only the thickness of the shell. 
Thus there is a relationship between the age of the shell and its thickness, and in 
comparisons both age and size must be considered. 
The direction of the rib is another point of alleged difference between the 
two varieties. It is charactized as " straight " in " patula " and " oblique " in 
" var. nuttalliV^ The rib is somewhat more oblique in young shells than in old ones, 
