248 MR. E. 8S. RUSSELL ON THE SHELL-GROWTH [ Mar. 2, 
second year, at the end of which the limpet is about 38 mm. long. 
The sequence of events during the third year is probably very similar 
to what it is in the second. Growth is even slower, and the 
limpet may increase only 5 mm. in length. The limpet possibly 
becomes full-grown at the end of this third year or at the end of 
the fourth year—at a length of a little more than 45 mm. 
Growth is almost at a standstill during the fourth year, and shells 
of 48 mm. and upwards may exhibit for long periods no growth 
at all, so that their age may be considerably more than four or 
five years. 
The distribution of lengths in a large random sample (Table IT.) 
shows that the limpets are subject to a considerable degree of 
elimination, the numbers in the first year-group (say 9-23-5 mm. 
for the month of July) being much greater than the numbers for 
the second year-group (say 24-38°5 mm.). The third and subse- 
quent year-groups are not shown separately by a random sample 
but merge into one another. The great number of shells 45— 
53°5 mm. long which occur in the sample show that growth at 
this size is practically stationary, for shells of this size represent 
several different year-groups. 
It is of interest to compare the life-history of the Tortoiseshell 
Limpet, demea testudinalis, as related by Willcox (11). It shows 
much analogy with that of Patella vulgata. Acmea testudinualis, 
it should be mentioned, reaches a length of only some 20 mm. 
The breeding-season in America lasts from the middle of April 
to the end of July. Boutan (1) says that “ la ponte de /Acmée a 
lieu en avril et en mai” on the French coast, and I have reason 
to think that the same holds for the Acemea of the Clyde. 
Willcox finds that the young dAcmea are 4-5 mm. long in 
September and early October, and that they attain sexual 
maturity under 10 mm., probably after one winter. 
In Plate XXXII. the first two rows show stages of about fifteen 
months’ growth, the second and third limpets in the second row 
showing the size at which sexual maturity is reached. The first 
two in the third row show the size of second year shells, the 
second (38°5 mm.) being from two to two and a half years old. 
The first limpet in the fourth row is probably a little more than 
three years old, while the other two in the row are full-grown 
limpets at least four to five years old. The two in the bottom 
row are veterans, the last one (61°2 mm. in length) being the 
largest limpet I have ever collected. 
Il. CHANGES IN SHAPE DURING GROWTH. 
About 1000 shells were collected at random during July 1903 
and July 1904 at the Southend of Arran, and the ratios of the 
various dimensions calculated for every shell. 
Table VII. gives the mean ratios in each group; JL stands 
for length, B for greatest width, H for height, Ab and Af are 
explained a few lines further on 
