This singular variety cannot be considered accidental, in the sense in which this term may be applied to some varieties: the larva 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 Scalpellum vulgare. But when other species of 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 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 the two last described fossils, namely, [B. varians] and [unguiformis], 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.
2. Sub-Genus—ACASTA. Pl. [9].
ACASTA. Leach. Journal de Physique, tom. lxxxv, 1817.
Compartments six; parietes and basis non-porose: basis calcareous, cup-formed, not elongated, attached to sponges, or rarely to the bark of Isis.
Distribution mundane; imbedded in sponges and the sponge-like bark of Isis.
This sub-genus, in one sense, is a very natural one, inasmuch as all the species are closely allied in essential structure, in general appearance, and in habit. On the other hand, in the structure of the shell, in all the characters derived from the opercular valves and animal’s body, [Acasta] cannot properly be distinguished generically from some species of [Balanus]; thus [B. navicula] and [cymbiformis] agree in the parietes and basis not being porose and in all other essential respects, differing only in the shell being more elongated in the rostro-carinal axis and in being attached to Gorgoniæ instead of to sponges; yet we shall see that [Acasta purpurata] lives imbedded in the bark of Isis, so that even the habit of being imbedded in sponges fails. [Balanus terebratus] would have been ranked as an [Acasta] had it inhabited sponges. On the other hand, some species of [Balanus] inhabit sponges, as is often the case with [B. spongicola], and always with [B. declivis]: but both these species are distinguished easily from [Acasta], the former by its porose walls and basis, and the latter by its membranous basis; it may, however, be reasonably doubted whether such differences ought to be considered as even sub-generic. The most important character of [Acasta] probably consists in the anterior ramus of the fourth pair of cirri, differing slightly in the arrangement of its spines, and in some other points, from the rami of the two posterior pairs of cirri,—a character not as yet observed in any other cirripede. Had not the genus [Acasta] been already founded and extensively admitted, certainly I should not have formed it; but considering the close similarity in habits, aspect, and structure, of the nine species of [Acasta], and considering the already large size of the genus [Balanus], I hope I may stand excused for admitting [Acasta] as a sub-genus.
General Appearance.—The shape varies from nearly globular to that of a somewhat flattened acorn, the orifice being often a little contracted from the inward curvature of the tips of the parietes. In [A. spongites], however, the orifice is generally widely open; and, on the other hand, in [A. sporillus], the orifice is reduced to a mere pore. The usual tint is pale reddish, but [A. purpurata] is purple, and [A. sporillus] purplish-brown. The surface is either smooth, or is shagreened with minute points, as in [A. sporillus], and [fenestrata], and in some specimens of [A. sulcata]; and in all the species, except [A. sporillus] and [fenestrata], many individuals are furnished with elongated, curved, sharp, shelly points, like those in var. spinosus of [Balanus tintinnabulum]. The summits of the radii, which are generally of moderate breadth, are more or less oblique; their surface is often marked by lines parallel to the basis, instead of by vertical lines corresponding with the lines of growth, as in most species of [Balanus]. The carino-lateral compartments vary in proportional breadth in the different species: in [A. sporillus], they tend to become rudimentary, and in this species (Pl. [9], fig. [9 b]) their basal margins, or rather points, do not reach down to the basis. The species are all small, [A. glans] and [undulata] are the largest, being sometimes .55 of an inch in basal diameter.