General Appearance.—The shell is in an almost rudimentary condition, and appears like a small white irregular star, imbedded up to its top in the skin of the porpoise. Out of this thin, star-shaped shell, a cylindrical, flexible, peduncle-formed body springs, which forms the main part of the animal; it is narrow where coming out of the central cavity of the star, but soon acquires its full diameter; at the upper end it has a reflexed hood, and hence is broader, and this has the appearance of forming a capitulum, like that of a pedunculated Cirripede. This pseudo-capitulum is formed by a membranous reflexed collar or hood, which is very narrow at the lower end of the orifice, close under the mouth, and becomes wider and wider towards the upper and carinal or posterior end of the orifice; hence the lower reflexed edge of the hood is only slightly oblique or even nearly transverse. The orifice leading into the sack is large, and nearly in the same straight line with the peduncle; it is a little hollowed out in the middle at the upper end, and on each side of this medial hollow, there is a small rounded projection or horn, not perforated, but hollow, as may be seen by turning up the hood and looking at its under side. These two little horns curiously bring to mind the ear-like appendages in Conchoderma aurita (Otion), but these latter are perforated, open into the sack, and point outwards. The peduncle-formed body answers, as we shall presently see, to the main part of the shell in Tubicinella, and the hood, as it would appear, to the lips of the sack-aperture, which project between its scuta and terga; of these valves there is not here a trace. The whole surface is smooth, and is formed by rather thin membrane, of an orange colour; but from the colour of the underlying corium, the whole appears of a dark chocolate red, the reflexed hood being rather lighter coloured. It is singular how closely the colour resembles that of some dark varieties of the above-mentioned Conchoderma aurita, and likewise of Anelasma squalicola, both pedunculated Cirripedes, having oceanic habits, and destitute, to a remarkable degree, like Xenobalanus, of shelly valves.

The largest specimen which I have seen was very nearly two inches in length: in this specimen the star-shaped shell measured, from extreme point to point, nearly a quarter of an inch in diameter, but the internal cavity only about one eighth of an inch. This latter measure gives also the diameter of the peduncle, where coming out of the shell; the diameter just beneath the hood, was in this same specimen rather more than a quarter of an inch, and therefore greater than the diameter of the points of the shell. The depth of the shell from the upper rim to the basal membrane, in one specimen which I measured, was only one twentieth of an inch, and this specimen had its pseudo-peduncle one inch and three quarters in length, consequently thirty-five times as long as the shell was deep.

Structure of Shell.—The almost rudimentary shell (fig. 4 b) consists of a small, thin, six-rayed disc, formed of six compartments, each of which, instead of being outwardly convex, as in ordinary Cirripedes, is deeply bowed inwards. The narrow sutures (s s) separating the six compartments, run along the middle of the six rays, each ray being composed of the bowed ends of the walls of the adjoining compartments. The rays are a little curved towards the carinal end of the shell. It is remarkable that the rostrum is smaller and less deeply folded inwards than the other compartments, and the lateral compartments are a little smaller than the carino-lateral compartments, which is exactly the reverse of what is usually the case. Only about four zones of growth have been preserved in any specimen, and consequently the shell is very nearly of the same diameter at the top and bottom; for the upper end of the shell is rapidly removed, as in Tubicinella, by the scaling off of the upper rims of the sheath, and by the disintegration of the walls. The zones of growth are commonly not piled exactly over each other, but rather obliquely, as represented in fig. 4 b. Each zone projects, forming a prominent, sharp, toothed ridge round the shell. In Coronula (Pl. 16, fig. 6) and its allies, the outer lamina of the wall is formed by the union, a little above the basal margin, of ledges running along the sides of the longitudinal septa. In Xenobalanus (Pl. 17, fig. 4 c) similar ledges are less perfectly joined, and apertures seem always to be left in transverse rows under the transverse toothed ridges, which latter are best seen in fig. 4 b. The apertures, of course, are covered by membrane. The transverse ridges are surmounted by knobs arising from the longitudinal septa; and the knobs themselves are capped by other little heads, which are not represented in the drawing. Owing to these projections, and to the prominence of the transverse ridges and of the longitudinal septa, the external membrane is attached so firmly to the shell that even with the aid of caustic potash it can hardly be separated.

The internal cavity of the shell is small: it approaches a hexagon in shape, with the rostral side very short, and the lateral sides curved inwards. It is lined by a rather thick sheath, which descends very near to the basal membrane; the sheath is divided into very distinct, successive convex zones of growth. The external membrane of the pseudo-peduncle is attached with remarkable strength to these rib-like zones of the sheath. The alæ (a in fig. 4 b) are represented by mere angular shoulders, received into very slight notches, and placed at the inner ends or entrances of the double walls, or rays as I have called them. With respect to the radii, they also are in an extremely rudimentary condition; but a thin layer of shell, apparently continuous and homologous with the sheath, extends from the sheath along both sides of each ray, and on the rostral side (whence the radius ought to arise), about half-way from the end of the ray, gives rise to a projection or ridge (d, in fig. 4 b) which runs from the top to the base of the shallow shell. From this longitudinal ridge, septa, parallel to the basis, extend to nearly the extremity of the ray or double wall. These represent the radii; but they never grow, so as to increase the diameter of the shell. These radii evidently correspond to the additional or pseudo-radii in Coronula, which in that genus lie between the parallel, as here, and folded parietes. Of the true radius, having the same thickness as the paries, I here saw only traces in an internal, very slight, longitudinal ridge running up the shell, close to the outer extremity of each ray or double wall.

The membranous Basis is united all round to a rather wide flap of membrane which is inflected from the outer surface of the shell. The cement-glands appear to be mere enlargements of the cement-trunks, which latter extend in two nearly straight and parallel lines. From each gland two cement-ducts proceed, one of which runs parallel to the cement-trunk.

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.