ACASTA. 33 
a fossil specimen of B. calceolus. Notwithstanding the above several strongly-marked 
characters, by which this variety differs from the ordinary form, there is a resemblance in 
colour and aspect, which, though difficult to be described, made me from the first suspect 
that the two were specifically identical. In no point of real structure is there any difference, 
excepting that, perhaps, the pores in the basis are here rather smaller; but this might arise 
from the little development of the peculiar basis. Having come to this conclusion, I was 
interested by finding a specimen (fig. 5e) in Mr. Wood’s collection, which had originally 
fixed itself (judging from the form of the basis) on a thick cylindrical stem, but which had sub- 
sequently grown on to an adjoining flat surface ; consequently, one side of the shell presented 
all the peculiar characters of the present variety, but not strongly pronounced, whereas the 
other side, at the rostral end, was undistinguishable from the ordinary form. The unequal 
development of the rostrum on the two sides was very striking, and clearly showed how 
great an effect could be produced by the nature of the surface of attachment. 
This singular variety cannot be considered accidental, in the sense in which this term 
may be applied in some cases: the pupa evidently fixes itself intentionally, in a certain 
definite position, on the branch of the coral (when a branch is chosen), exactly as in the case 
of Balanus calceolus, or Scapellum vulgare,—species which always live attached to branches. 
But when other Balani occasionally fix themselves on branched corals, their position seems 
to be accidental and unsymmetrical; thus among the symmetrically elongated specimens of the 
present species, I found one specimen of Balanus bisulcatus, which had evidently been 
attached in an almost transverse position toa branch, and had thus become much distorted ; 
so, again, I have seen specimens of the recent B. amaryllis attached irregularly to a Gorgonia, 
in the midst of the symmetrically elongated shells of Balanus navicula, an ally of B. calceolus, 
This variety does not seem to attain so large a size as the ordinary form. 
Affinities.—Vhis species is allied to B. unguiformis and B. varians, but is perhaps more 
nearly related to the recent B. a//ivm, an inhabitant of the Barrier Reef of Australia. The 
longitudinally folded variety (6) can hardly be distinguished by external aspect, or even by 
the opercular valves, from B. crenatus ; but when the shell is disarticulated, the porose 
walls and non-porose basis of B. crenatus, allow of no mistake in the diagnosis of the 
two species. 
Sub-Genus—Acasta. 
Acasta. Leach. Journal de Physique, tom. Ixxxv, 1817. 
Valve teste 6 ; parietes et basis non porosa ; basis calcarea, cyathiformis, non elongata. 
Valve operculares inter se articulate, subtriangulares. Spongiis, aut rard Isidis cortict, 
affira. ; 
Compartments six; parietes and basis non-porose: basis calcareous, cup-formed, not 
5 
