1909. ] OF THE LIMPET PATELLA VULGATA. 243 
now in July of an average length of 17-18 mm. These facts 
taken together seem to Vena us in placing in the first year- 
group the limpets, Nos. 1-5 (Table I.). These reach a length of 
about 15 mm. by the third week in May. Nos. 6-31 form a 
natural group, since they were all found in a small area of a few 
square yards and present no abrupt difference in Sse. Nos. 6-23 
are quite clearly of the same year-group as Nos. 1—5,and it seems 
safe to include also in this first year-group Nos. 24— 31 and 32-36. 
No. 29 for example, though 19-6 mm. long on May 25th, is only 
29°2 mm. long on Dec. 31st, a length which is reached on that 
date by No. 25 which was only 16-4 mm. long on May. 25th. 
One must allow for a good deal of variation in the rate of growth 
as well as for the fact that each year- group may include limpets 
differing as much as six or seven months in age, since the breeding- 
season is of that length. Nos. 27 and 28 illustrate how one 
limpet may differ from another in rate of growth, so also do Nos. 3 
and 4. On the other hand the rate of growth often exhibits 
surprisingly little variation, as is shown by Nos. 12, 13, and 
14, 17 and 18, 24 and 25, and others. 
The placing of Nos. 37, 38, and 39 in the first year-group is a 
little doubtful, but is I think on the whole justifiable. 
The mean values have been calculated from the month of May 
onward, and from them we can infer that a limpet which has 
come into existence say In September will reach a length of about 
15 mm. in the following May, 18 mm. in June, 20 mm. in July, 
24 mm.in August, 26 mm. in September, 27 mm. in October, 
28 mm.in November, and by the end of the year will attain on an 
average a length of 29 mm. 
These figures are of course only roughly approximate and are 
valid only for the particular locality where the limpets grew. 
Limpets at the Southend of Arran seem to reach a length of only 
17-18 mm. by the month of July. 
Second Year-Growp.—Given the first year-group it is a simple 
matter to determine the second year-group. On December 31st, 
1908, the first year-group had a certain range and a certain mean 
value, and the second year-group must necessarily have had on 
December 31st, 1907, approximately the same range and the 
same mean value. The limpets assigned to the second year-group 
(Nos. 40-65) have been selected on this principle. Their range 
of size on December 31st, 1907, 1s from 22 mm. to 34:4 mm., 
and their mean 28°83 mm. The mean value of the first year- 
group on December 31st, 1908, was 29°1 mm. and the range from 
24:5 mm. to 33°8 mm., but if No. 7 had survived till December 
31st the range would have been from about 21:5 mm. up to 
33°38 mm. Theaverage size on December 31st, 1908, in this year- 
group is 38 mm. 
Subsequent Year-Groups.—lt is not possible to arrange the 
limpets of 35 mm. long and upwards in year-groups with any 
certainty. The arrangement adopted above seems the most 
satisfactory, but it is only tentative. 
