No. 461.] BIOLOGY OF ACM HA. 327 
tions probably at least as variable as those of the Massachusetts 
waters. lf now we examine their size with reference to their 
habitat, we find that the limpets of Eastport are large only 
when living at or near the low-water mark of spring tides so that 
they are rarely or never left uncovered by the sea, and that 
higher up on the beach the animals though no less abundant are 
of smaller size, no larger in fact than with us. "We find also 
that in Massachusetts there is no marked difference in size 
between limpets which are continuously submerged and those 
which live between tide-marks. The conclusion is therefore 
forced upon us that size in these animals is correlated not neces- 
sarily with a low but with an equable temperature. The advan- 
tage of uniform thermal conditions was long ago pointed out by 
Mobius and Semper. 
In the late autumn, limpets in the neighborhood of Boston 
either altogether disappear or become very scarce, reappearing 
in numbers in March or April. In Eastport, however, they 
remain near the shore for the entire year. Their disappearance 
from the Massachusetts coast is probably due to a retirement 
to deeper water; this is, I am told by Professor S. I. Smith, the 
practice among several of our littoral animals. Limpets will 
stand a good deal of cold; Miss Annie Sullivan, an observant 
and intelligent collector who is familiar with the animals at East- 
port, tells me that she has revived them after ice had formed 
within the shell and found them apparently no worse for the 
experience. It seems very probable therefore, that when they 
withdraw in the autumn they go only a little below tide-mark, 
thus avoiding the very considerable daily variation in tempera- 
ture to which they would often be subjected if they remained 
above the low-water line. 
The distribution of limpets as given above is taken from 
Smith and Verrill's * Invertebrate Animals of Vineyard Sound," 
a work which was published nearly thirty-five years ago. Iam 
not aware that any investigations have been undertaken since 
that time which would show whether it is still correct. Forbes 
and Hanley described this species in 1853 as having, at least 
in Great Britain and Ireland, a tendency to migrate southward 
and Forbes had already described its sudden appearance nearly 
