246 MR. E. 8S. RUSSELL ON THE SHELL-GROWTH | Mar. 2, 
October 1:1 mm., during November ‘6 mm.,and during December 
‘T mm. ‘The percentage increase falls even more rapidly. 
But it is in the second year-group (where there is little 
variation in the rate of growth due to age) that the seasonal 
variation is shown most clearly. During the first three months 
of the year growth is practically at a standstill, then during the 
summer a slow increase In size is manifested, but after October 
growth almost ceases again. It should be remarked in passing 
that the jump from 33-7 mm. at J uly 30th to 36 mm. at Sep- 
tember 2nd does not signify that growth is specially active durmg 
August. The increase is due to the introduction into the group 
on September 2nd of the large shell No. 65 and to the reappearance 
of No. 64. 
The seasonal variation in the rate of growth may be proved in 
another way. In Table VI. the percentage growth during the 
month of August is tabulated for limpets of all sizes. 
It is easy to calculate from these data the number of days 
which a limpet would take to grow from 10 mm. to 15 mm., from 
15 mm. to 20 mm. and so on, if it grew as quickly during the 
other months of the year as it does during August. Using the 
formula 
OG 4b IP 
oa LOO y sa 
where O = original length, F = final length, P= percentage 
increase per 30 days, and 2 = number of Tmo (of 30 days) 
taken to grow from O to F, we obtain the following results :— 
Time taken to grow Days. Sums. 
‘hove 10) wawTeY, (HO) IS) WITN, 55545 Joaccncne one 69°26 69°26 
15 Fe EO Rest ae ie ho Peele 59°71 128'97 
20 pie ay ae pre BoC 57:18 18615 
Ns acon’ Cilia teen mete at os 82°08 268°23 
30 IS OV ieee aise ae Sore 93°87 362°10 
35 pO aren ie ee vt Rae eee 100°77 462°87 
40 sed eA edna re ee 132715 595°02 
45 Sp OOMNae iitosacansedadace 30633 901°35 
Suppose we start with a shell which is 10 mm. long on 
January lst. If growth during the first two or three months is 
as rapid as it is during August, the shells in the first year-group 
should be 15 mm. long on March 10th, 20 mm. long on May 9th, 
25 mm. long on July 5th. But actually, as is shown in Table I., 
they reach a length of 15 mm. only by the end of May, and 
20 mm. some time towards the end of July. Hence growth during 
late winter and early spring is slower than during autumn. The 
same method might be applied to show that growth is again 
retarded on the approach of winter, but one more application 
of the method will suffice. If we take the second year-group 
and start with a shell 30 mm. long on January Ist, it will, if it 
erows at summer rates, reach a length of 35 mm. on April 4th, 
