The pseudo-peduncle, forming the main part of the animal, has, as far as external appearance is concerned, been sufficiently described. The part forming the hood apparently answers to the protuberant lips of the operculum, and the lower part to the sub-cylindrical shell of [Tubicinella]; both shell and peduncle in the two genera being wider at top than at bottom. If in imagination we chip away (an action always in progress) the whole upper part of the shell of [Tubicinella], leaving only two or three zones of growth at the base, we shall convert it into a [Xenobalanus], with every internal part and organ occupying the same relative position: for it should be borne in mind that the shell of [Tubicinella] is lined close down to the basis by the opercular membrane, and this is strictly comparable with the outer membrane of the pseudo-peduncle of [Xenobalanus]. The body, as in [Tubicinella], is attached in a vertical position, with the longer axis of the thorax and of the much elongated prosoma extending in the direction of the longitudinal axis of the pseudo-peduncle. The point of attachment of the body to the lower part of the hood is much elongated, but presents the usual muscles running to near the base of the labrum and embracing the prosoma. The adductor scutorum muscle is well developed, and is placed close beneath the mouth, where the collar or hood commences folding over: hence, no doubt, the lower end of the large and long orifice into the sack can be closed by the adductor; but the upper end probably can only be blocked up by the outer surfaces of the curled cirri. The scuta, I may add, if such had existed, would certainly have covered the point where the adductor muscle is attached to the exterior membrane. The sack extends down to within the almost rudimentary shell: the tunic lining it is unusually strong; indeed, in the reflexed hood-like portion, it is as strong as the properly external membrane. The latter seems to be moulted in large strips, and not in a single piece, like the opercular membrane in [Balanus]: just above the shell, fragments of three or four of these outer coats are retained by their very firm attachment to the sheath. The two layers of corium, lining the outer membrane of the pseudo-peduncle and the inner tunic of the sack, instead of being, as usual, united almost continuously together, stand some little way apart, and are connected by longitudinal septa; hence, in a transverse section, especially of the lower part, the sack is surrounded by an irregular ring of square tubes of corium. The muscles surrounding the sack and imbedded in the inner fold of corium, are very thinly spread out; they branch, and even sometimes cross each other; they are more numerous at the carinal and rostral ends, but certainly cannot be said to form six (or five) bundles, as in all other sessile cirripedes. Some of the fasciæ extend down to the very basis, and some up to the summit, to near the two little horn-like projections. I could not perceive any transverse striæ on these muscles. Altogether, they are very weak, and cannot have much power in moving the whole peduncle-like body.
Branchiæ.—These are largely developed: they are attached to two approximate, longitudinal, fleshy crests, which extend more than half-way down the sack, along the carinal margin. Each branchia is double, the two folds being united where attached in a transverse line across the sack, on a level with the attachment of the body. The inner fold is much smaller than the outer; not extending half so far down the sack, and not extending so far transversely; it is also hardly at all plicated. The larger and plicated fold extends down considerably below the lower end of the prosoma, and altogether fully equals one third of the entire length of the animal, measured from the shell to the summit of the orifice. Both folds are formed of very delicate membrane.
Mouth.—Labrum unusually prominent, as measured from its basal margin to the crest, which is but slightly notched, hairy, and without teeth. Palpi broad, heart-shaped, clothed on their inner sides by a thick brush of spines, which here, as on the other gnathites and cirri, are almost all doubly serrated. On the outer margin of the palpi there are a few longer spines. Mandibles villose, with five teeth, of which the fifth is very small and of irregular shape: the inferior angle is broad and pectinated. There are no intermediate teeth between the second, third, and fourth teeth, as in the three foregoing genera. Had I not known that the lower main mandibular teeth were always laterally double in the [Balaninæ], and had I not observed how obscure this structure was in [Coronula] and [Tubicinella], I should have overlooked the merest vestiges of double teeth in the present genus; indeed, in some specimens the teeth seemed to be absolutely single. The maxillæ are villose: their edge exhibits a trace of being notched under the two great upper spines. The outer maxillæ are bilobed, but not very plainly: between these organs there is no little prominent mentum, as in the three previous genera.
Cirri.—The cirri are short, particularly the three anterior pairs. The segments in all are singular, from being so much compressed, so short, and of such great breadth; they are protuberant in front. In the second and third cirri, the broad lateral faces of the segments, with the exception of the posterior face of the posterior ramus, are almost bare of spines. In the three posterior pairs of cirri (Pl. [29], fig. [6]), the segments are protuberant in front, and support three pairs of short thick spines, with an intermediate tuft; the dorsal tufts are unusually small: their pedicels are remarkable from the upper segment, and the upper part of the lower segment, being produced into a rounded protuberance, dotted with spines: I have met with a similar structure only in Scalpellum vulgare. In the third pair (fig. [5]) there is only a trace of this structure; and in the second pair the anterior margin of the pedicel is straight, and clothed with three tufts of bristles. The pedicel of the first cirrus is very broad, and clasps the mouth.
The cirri and mouth are dark chocolate red, like the outside of the animal and the upper part of the sack. The thorax is redder and paler. The four posterior articulations of the thorax are straight and transverse; the next segment, or that corresponding with the second pair of cirri, is slightly inflected, in the usual way, towards the prosoma. The prosoma is pale coloured, extraordinarily elongated, and bluntly pointed; it extends down (see the dotted outline in Pl. [17], fig. [4 a]) about one third of the length of the whole animal. The orifice leading into the acoustic sack forms a freely depending little point beneath the basal articulation of the first cirrus. The stomach in the uppermost part is deeply and closely plaited longitudinally, but has no cæca; it runs down (externally coated, as usual, by the testes) to the lower point of the prosoma, and is then doubled back on itself, so that it is very long.
Generative System.—The probosciformed penis is short and thick, and covered with very minute tufts of bristles: there is no knife-edged projection at its dorsal base. The vesiculæ seminales are much convoluted and of great length. The ovarian tubes form a small sheet within the rudimentary shell, in the normal position, over the basal membrane; and likewise higher up between the two folds of corium surrounding the pseudo-peduncle; they do not, however, appear to occur round the lower part of the peduncle: they extend highest on the rostral and carinal sides, and lowest on the two lateral faces. The ova are wonderfully numerous; they are 15/2000ths of an inch in length: they form, instead of two thin lamellæ, two almost cylindrical packets, which are held together by most feeble membrane. Each packet, in the upper part lies between the two folds of the branchiæ; and in the lower part, is embraced only by the larger outer fold. The two packets of eggs sometimes cohere together at their lower ends.
Imbedment.—The shell is imbedded up to its summit, but the shell is very shallow. The imbedment seems due either to the compression of the epidermis of the porpoise, or to its formation beneath the shell having been checked; the outline of the true skin under the dark-coloured epidermis is not in the least affected. The epidermis fills up the bay-like spaces formed by the inwardly folded walls, and firmly adheres to them.
Affinities.—This genus presents very varied affinities to [Tubicinella], [Coronula], and [Platylepas]. To the latter it is more especially allied in the compartments, being singly folded inwards, though the sides of the folds are not here closely pressed together, as in [Platylepas]: in both these genera the fold is less deep in the rostrum, of which fact, in [Platylepas], the final cause is evident, but here there seems no cause, excepting the simple one of affinity. [Xenobalanus] is further allied to [Platylepas], in the lesser size of the inner fold of each branchia, compared with the outer, and in the structure of the cement-glands, and to a certain extent in that of the sheath. To [Coronula] the special alliance is shown by the remarkable character of the pseudo-radii lying between the parallel and adjoining walls, and in the general character of the cirri: in [Coronula] we have the terga sometimes quite aborted, and the scuta of small size, thus exhibiting a tendency to the entire absence of opercular valves, so remarkable in [Xenobalanus]. To [Tubicinella], the alliance is still more plainly shown in the external shape of the whole animal, wider at top than at bottom,—in the opercular membrane descending almost to the very base—in the relative positions of the different parts and organs—in the upper end of the shell continually scaling off—in the prominence of the transverse external ridges—in the sheath being divided into successive zones of growth, and being prolonged nearly to the basal membrane—and in the edges of the alæ being smooth. Internally, the resemblance is also plainly shown, in the strength of the internal tunic of the sack—in the branchiæ springing from two approximate fleshy crests—in the freely depending acoustic orifices—in the form of the thorax and prosoma—and consequently of the alimentary canal.