Dr. F. Miiller on Balanus armatus. 399 
y. & vi. Two of the shells from the Papillina seated on 
the third empty shell; v. has the carina, and vi. the rostrum 
turned towards the orifice of the subjacent shell: in the former 
the rostrum is more than four times as long as the carina, in 
the latter the carina nearly three times as long as the rostrum ; 
but the planes of the orifices of all these shells are nearly 
parallel. 
vu. Shell seated on a Purpura. 
Scuta.—The scuta are very narrow; the occludent margin 
is nearly or fully twice as long as the basal margin ; the ter- 
gal margin is a little shorter than the occludent margin. The 
apex is usually slightly curved upwards; the outer surface is 
covered with strongly projecting lines of growth, and with 
from one to six longitudinal rows of pits, which are generally 
very deep, and frequently of considerable width (fig. 2). In 
twenty-eight animals taken at random, there were in the 
lower part of the scuta :—once, two on each side; eight times, 
three; eight times, four; once five, and once six rows; fur- 
ther, six times, three rows upon one scutum and four on the 
other; twice, four on one side and five on the other ; and, lastly, 
once, five on one side and six on the other. A single row of 
pits was seen by me only on the two animals adhering to 
Purpura. In the inside of the scutum there is an articular 
ridge, not of great breadth, which reaches beyond the middle, 
or even to the lower third of the scutum, and terminates there 
in a rounded end or in a small point. The adductor ridge is 
inconsiderable, and extends scarcely further downwards than 
the articular ridge. Sometimes there is a very fine, sharp 
longitudinal ridge between the articular and adductor ridges. 
For the musculus depressor lateralis there exists a pit which 
is usually narrow and deep. In the larger animals especially 
the scuta are often of remarkable thickness. 
Terga.—These agree perfectly with the descriptions given 
_by Darwin of Balanus trigonus. The six to seven ridges for 
the musculus depressor* never extend beyond the basal mar- 
* Darwin’s statements as to the action of the three pairs of muscles 
which descend from the operculum towards the base of the shell appear 
to me, from what I have observed, especially in Tetraclita porosa, not to 
be quite correct. According to Darwin, the operculum is opened by the 
depressores scuti laterales; sudden contractions of the depressores ros- 
trales probably cause the strokes which the animal gives with the beak- 
like apices of the terga; by the common contraction of the three pairs 
the ee is held down with surprising force; the operculum can 
only be raised by the pressure of the body against the base. (Darwin, 
‘ Balanidee,’ p. 62.) 
If we attempt to cut the operculum of Tetraclita porosa out of the shell, 
the knife will find a free passage everywhere except in two opposite 
