Dr. F. Miiller on Balanus armatus. 397 
hundreds upon hundreds of different forms to exhaust the va- 
rieties of these Balani seated upon Carijoa; nevertheless in 
these, and likewise in the shells seated upon rocks, a steeply 
conical form predominates. .The two shells observed upon 
Purpura were flatter than usual; their walls were less steep, 
and their base larger in proportion to the orifice. 
The surface of the walls is usually smooth, rarely furnished 
with inconsiderable longitudinal ribs; the shells attached to 
rocks generally have stronger ribs. The colour of the walls 
is sometimes quite pale; sometimes they are striped with a 
lighter or darker dingy brownish purple. The radii usually 
exhibit a more or less distinct dingy purple colour. Not un- 
frequently there is a remarkable difference of colour between 
the two sides of the same shell; and if we may imagine that 
the influence of light has something to do with this, this ex- 
planation is not applicable to a group of three shells of which 
the lowest and largest is unusually dark-coloured, the second, 
sitting upon this, is almost white, and the third and youngest, 
which adheres to the second, has particularly distinct whitish 
ribs, and between these pale-brown streaks. ‘The sheath is 
pale ; the opercular pieces partly pale, partly dark, but usually 
reddish, at least towards the apex. 
I never found the epidermis preserved upon the radii, and 
rarely in traces upon the lower part of the walls; but I possess 
an example from a Reniera the walls of which are still com- 
pletely covered with a yellowish membrane, and the radii of 
which are, moreover, distinguished by their white. colour. 
But, different as the shells of Balanus armatus may be in 
their form and colouring, they all agree completely in the pe- 
culiar form of the mouth, which reminds one of that of B. tri- 
gonus, though, unlike that of the latter species, it is always dis- 
tinctly toothed*.. The radii are always oblique, especially those 
_of the rostrum ; their free margins usually form with the wall of 
the rostrum an angle a little under, and with that of the lateral 
piece a little over 60°, meeting the ale of the lateral piece 
about in the middle. In like manner the margins of the radii 
of the lateral piece and of the ale of the carino-lateral piece 
meet each other about in the middle, whilst the margins of the 
alee of the carina only meet the radii of the carino-lateral pieces 
close to the walls of the latter. Like the denticulation of the 
orifice, we find, as a second peculiarity in all well-preserved 
shells, that the rostrum isa little bent inwards at the orifice. 
* The sole exception (and this is probably only apparent) is furnished 
by the three shells which I found thrown up on the shore. Their mouths 
are entire and toothless; but I believe that they only lost their teeth 
during their rolling in the sea and surf. 
