396 Dr. F. Miiller on Balanus armatus. 
which are seated close together upon the tube of a Serpula 
(Hupomatus floribundus, ¥. Mill.), which bears, close to them, 
two shells of B. tmprovisus, var. assimilis. This, which is the 
commonest of all the Balanidee here, sometimes even occurs in 
Reniera as the companion of B. armatus. 
General appearance.—The shell of Balanus armatus (fig.1), 
as regards form and colour, is very variable in its general 
appearance. tor the most part, in this, as in other species, 
this difference 1s caused by the support upon which the animal 
has domiciled itself. Hence the most regular shells are gene- 
rally those imbedded in soft sponges, the development of 
which never meets with any obstacles. They are usually 
found here of a steeply conical form sometimes nearly eylin- 
drical, the longitudinal diameter (from the rostrum to the 
carina) generally rather greater than the transverse, the ros- 
trum and carina nearly of equal height, the base always con- 
cave, and generally in a high degree. In this respect, there- 
fore, B. armatus agrees with the allied B. spongicola, and 
differs, like the latter, from the sponge-haunting Acaste, in 
which the base is strongly convex. But even in Reniera very 
divergent forms are not wanting. I have seen shells in which. 
the rostrum was only half as high, and others in which it was 
twice as high, as the carina. oe 
The form of the shells seated upon the Cardjoa is particu- 
larly variable. It differs according as they are attached longi- 
tudinally, or transversely, or obliquely to the stem of the polype, 
which usually forms a deep furrow. in their base. ‘This is 
elongated in the direction of the furrow, which, again, has an 
influence upon the whole shell, so that even in its middle the 
breadth varies from two-thirds to four-thirds of the length. 
Not unfrequently the separate pieces of the shell are of very 
different heights, all the pieces of one side being sometimes 
twice as high as those of the other. Rarely the furrow of the 
base is closed, so as to form a complete tube. On one occasion 
I found Balanus armatus adhering to the apex of a branchlet ; 
and in this case the base formed a conical tube round the 
branchlet, longer than half the height of the shell; the dia- 
meter of the base was only half the length of the orifice. In 
other cases, again, the base is more than twice as long as the 
orifice. Shells inflated in the middle also occur. A particu- 
larly remarkable form was seated transversely upon the stem 
of a Cartyjoa. The rostrum and carina are unusually broad, 
almost equilaterally triangular; they embrace the stem and 
meet together beneath it upon one side in a sharp edge. The 
walls of the lateral and carino-lateral pieces, on the contrary, 
are quite narrow strips. But it would be necessary to figure 
