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. 5<?) 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 Scapelhm vulgar e, — species which always live attached to branches. 

 But when other Balani occasionally fix themselves on branched corals, their position seems 

 to be accidental and unsyinmetrical; 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 to a 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. — This species is allied to B. unguiformis and B. varians, but is perhaps more 

 nearly related to the recent B. allium, an inhabitant of the Barrier Reef of Australia. The 

 longitudinally folded variety (b) 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, torn, lxxxv, 1817. 



Valva testa 6; parietes et basis non porosa ; basis calcarea, cgathiformis, non elongata. 

 ValvcB operculares inter se articulates, subtriangulares. Spongiis, aut rarb Isidis cortici, 

 affix a. 



Compartments six ; parietes and basis non-porose : basis calcareous, cup-formed, not 



5 



