38 
CALIFORNIA SEA-SHELLS. 
scar, within the shell, is very evident. This 
species is thick and solid, white, more or less flat- 
tened and distorted, with rough lines of growth 
and indistinct radiations. Breadth, from one- 
half to three-quarters of an inch. The members 
of this genus build a shelly layer under the foot, 
which also shows the peculiar muscle-scar. 
There is a good deal of uncertainty respecting 
the number of our California species, as they are 
variable in appearance in the different stages of 
growth, and different species appear to run into 
each other. These doubtful questions present 
a good field for study. 
Scurria mitra, Escli., Fig. 2, PI. X, has a pure 
white, conical shell, which is sometimes worn as 
the drop of an ear-ring. It is said to live on the 
roots of sea -weeds. I found one living specimen 
on a rock, at extreme low tide. It can easily be 
distinguished from other shells, by its sharp con- 
ical form, pure white color, and nearly circular 
outline. Dead shells are common on the beach ; 
common length, one inch. 
Nacella incessa , Hds., Sea-weed Limpet, Fig. 
3, PI. X, lives on sea-weeds. It is oblong, coni- 
cal, horny in appearance, brown, thin, one-half 
to three-fourths of an inch in length. 
Most of the limpets live near the shore, be* 
tween high and low water marks. Almost all of 
our species may be collected with moderate 
E ains ; many of them are very pretty, and they 
ave one advantage for the collector, namely, 
they are easily cleaned. They may be dislodged 
from the rock by a sudden lift, with a broad blad- 
ed knife, but if previously disturbed, they cling 
