252 THE SHELL-GROWTH OF THE LIMPET PATELLA VULGATA. [ Mar. 2, 
have been at work on other shells 15-42°5 mm. in length—-that 
possibility always remains. But for this group the action of 
natural selection between the lengths of 15 mm. and 42°5 mm. 
is excluded. Hence all the changes taking place during this 
period of growth are independent of selection. Now the values 
obtained for the ratio of length to breadth approximate very 
closely to those obtained for Groups 15-20 mm. and 40-45 min. 
of the large series (Table VII.), and though the value for the 
breadth of the 40—42°5 shells at 15-17-5 mm. long (1°26) falls some- 
what below the value of the ratio in Group 15-20 mm. (1°27), 
and might lead one to think that these shells were a selected lot, 
yet the value of the ratio for the 60 shells 15-17°5 mm. in length 
is even more divergent, being only 1:24. The differences are 
clearly due to the difference in environment, the series of which 
the ratios are tabulated in Table VII. having been collected in the 
Southend of Arran. One objection may be made to these results, 
and that is that shells of 40-45 mm. are usually very much 
abraded towards the apex, and consequently the measurements 
made on them at a length of 15-17°-5 mm. cannot be depended 
upon. This objection has some weight, and to corroborate my 
results I have made measurements of 100 “smooth” shells 
25-35 mm. in length (75 of them being 25-30 mm. long), which 
were carefully selected from over two hundred very kindly collected 
for me by Mr. David Baird, M.A., from the shore where my 
original set of 1000 had been gathered. The shells selected were 
those which had suftered practically nothing from weathering, 
and I am certain that the results they give are not appreciably 
influenced by the effects of abrasion and erosion. The ratio ata 
length of 15-17:5 mm. on these shells was 1°22, and at a length 
of 24-26°5 mm.it was 1:19. The change in this ratio may quite 
certainly be set down toa “law of growth” and not to selection. 
This change comes about in approximately four months’ time. 
Summary. 
The breeding-season extends from July to January. 
Sexual maturity is reached at a length of 20-25 mm. 
An average size for a limpet of the last season’s brood in 
January or Februaryis 10 mm. At the end of the first year 
it may be 29 mm. long. Probable sizes at the end of the 
second and subsequent years are 38 mm., 44 mm., 48 mm., 
53 mm. Shells over 50 mm. may be considerably more than 
five years old. 
(4) Sexual maturity is reached in the first year, and when the 
limpet is only half-grown. 
(5) The rate of growth decreases with age and maturity. It is 
slower during the colder months of the year. 
(6) Considerable changes take place in the ratios of the shell- 
dimensions during growth. 
(7) These changes are probably in large part the expression of 
“laws of growth,” and not due to natural selection. 
Fa a i 
(SG) (SS) = 
VSS 
