Chapter XXIV: Introduction (20)

_Radii._--The radii are very wide in _C. diadema_. In all the species their summits are square or parallel to the basis. Their internal structure is remarkable: as the walls in this genus are extremely thin, so are the _proper_ radii, for in fact they consist in this and all cases, as we know, of one margin of the wall modified by its lateral growth against the opposed compartment. But as the radii in Coronula are not plicated, like the walls, the shell would have been excessively weak along the six lines of suture, had not the radii been strengthened by numerous sinuous plates, springing from the inner lamina of the proper radius, and running downwards, attached to the folded wall of the compartment to which the radius belongs, and with their free edges pressed against the folded wall of the opposed compartment. These plates give out short transverse denticuli, making altogether a beautiful structure, as is best seen in Pl. 16, fig. 3, but also in fig. 2 and 4, and _d_ in fig. 1. In the section, fig. 7, the proper radius (_d_) is seen to be continuous with the wall (_e_), and to be very thin, in fact forming but a small portion of the compound radius: it is formed of an outer and inner approximate lamina, separated by septa, which are nearly horizontal, and which consequently cannot be shown in the transverse section fig. 7. The outer lamina of the radius is imperfect, or does not reach quite to the suture, leaving the septa a little exposed (imperfectly shown in Pl. 16, fig. 3), exactly as is the case with the outer lamina of the parietes, at the basal margin of the shell. In all common Balanidæ, a ribbon of corium runs up each of the six sutures, and sends in fine threads between the septa of the radii, but here a thread of corium runs up a minute, cylindrical pore, situated on the line of junction between the radius and the wall whence it arises; and from this longitudinal thread the finer threads spring which pass between the horizontal septa of the proper radius: this cylindrical pore is rather large in _C. balænaris_, but excessively small in _C. diadema_ (see a black dot (_d′_) in section, fig. 7), and is solidly filled up in the upper part of the shell. The plates (fig. 3) which run down from the inner lamina of the proper radius, and form the greater part of its thickness, are occupied by fringes of corium, extending up from a ribbon of corium, running along each suture, like those which run along the bases of the spoke-like folded walls. This difference in the origin of the ribbons of corium, occupying the interspaces between the plates and the pores in the proper radii, shows the essential difference between the latter and the thick inner portions of the compound radii. In _C. balænaris_ (fig. 3), the compound radius extends from the outside of the shell to the sheath and to near the basal edges of the folded walls: in _C. diadema_ (fig. 2), it does not extend so far inwards and downwards; and in _C. reginæ_ (fig. 4), even still less so. It ensues from this circumstance that when, in these two latter species, a transverse section is made across the middle of the shell, a large chamber (_v_, fig. 7), occupied by the ovaria, is found on each line of suture (_s s_), separating the radii (_d_) and alæ (_a_).

The _Alæ_ in _C. diadema_ (section, Pl. 16, fig. 7, _a′_) and _C. reginæ_, are of remarkable thickness, nearly equal to that of the radii, and in _C. balænaris_, of considerable thickness; this is evidently to give strength to the shell, which is weakest along the lines of suture, notwithstanding that the radii have been specially thickened. The edge of each ala presents a miniature resemblance of the edge of the radius, namely, a central ridge sending off on both sides sinuous plates, themselves denticulated. In _C. balænaris_, the ala rests almost entirely on the inner surface of the compound radius; but in the other two species, in which, as already stated, the radii and alæ are separated by chambers, the ala rests on a plate (_c′′_ in fig. 1, 4, and 7), which extends from the top of the radius to the bottom of the sheath, narrowing downwards (_c′′_, fig. 1), and is a specially developed portion of the sheath for the radius to rest on. In _C. diadema_ and _reginæ_, the sides of the folded walls, at the ends of the chambers (_v_, in section, fig. 7), are strengthened by the deposition of layers of shell in connexion with the sheath. The sheath extends close down to the basal membrane in _C. balænaris_, and does not project freely: in the other two species it depends freely, but does not run quite so low down. I have only further to remark, that the sutures, though very strongly united, are not calcified together; for they easily separate after the action of caustic potash.

The _Basis_ (Pl. 28, fig. 1 _a_-1 _c_) is membranous and is formed of concentric slips: each slip has eighteen angles, corresponding with the open ends (see Pl. 16, fig. 5) of the eighteen folds of wall. A ray of membrane runs under each of these folds, being prolonged from the basal membrane; but these rays can hardly properly be called parts of the basal membrane, for they split at each period of growth along the middle, and the two halves are drawn from under the basal edges of the walls, and thus come to invest their outer surfaces. The basal membrane, and the whole cementing apparatus, which is much simpler than in Balanus, has been fully described (p. 135) in the Introduction.

_Opercular Valves._--In the sessile cirripedes hitherto examined, the four opercular valves are inclined towards each other, and nearly fill up the orifice of the shell, being united to the walls by a more or less narrow circular border of membrane; this membrane being attached rather low down to the sheath. In Coronula the opercular membrane is stretched like the skin of a drum, almost horizontally across the top of the shell; it is, however, generally attached to the sheath rather lower down at the carinal than at the rostral end: hence the animal's body, as remarked by Burmeister, is attached almost horizontally; but this we shall see, under Xenobalanus and Tubicinella, cannot be considered a character of much importance. The valves are quite small compared with the opercular membrane, and certainly are of little functional importance. The scuta in _C. balænaris_ and _diadema_ (Pl. 15, fig. 3 _b_) are sub-triangular; but the under or growing surface is elongated and arched. These valves stand almost at the rostral end of the orifice, instead of on each side of it. In _C. balænaris_ the terga are small, with the under surface oval: in _C. diadema_ they are either quite aborted, or are represented by a barely visible plate of shell, parallel and close to the tergal margin of the scutum. The aperture leading into the sack is formed by a nearly medial slit of considerable length, furnished with irregularly protuberant, inwardly inclined lips. These lips are formed by the development of an inner fold or crest of membrane, which can be just detected in most Cirripedes: the lips include a double fold of corium, and are covered by a delicate tunic, continuous with that lining the sack, and homologically continuous with the opercular membrane. The opercular membrane is very thick, tough, and yellowish; it is, in parts, finely plicated in lines radiating from the apices of the valves: these plicæ consist of membrane in an altered condition, being harder, more horny, and of a browner yellow; the plicæ are large at the rostral end of the scuta, and projecting beyond these valves, they afford attachment to the rostral depressor muscles. Rims of similarly modified membrane (Pl. 15, fig. 2 _b_) connect the scuta and terga together. The rims and plicæ are occasionally moulted together with the opercular membrane. In almost all hitherto described Balanidæ, a new opercular membrane is formed at each period of exuviation, and as soon as formed, the old one is generally moulted, together with the other membranes of the body: the case is very different in Coronula, in which it is evident, from the lines of growth on the valves and sheath, that a new membrane is formed only at rather long intervals, and that it is formed in some degree extensible, so as to allow of some growth in the shell. Two or even three of these membranes are retained at the same time, one over the other; and thus, by their joint thickness, they afford protection to the included animal's body, and compensate for the smallness of the opercular valves. In a large specimen of _C. balænaris_, two inches in diameter, there had been formed, since its existence as a very young shell, not more than eight opercular membranes, whereas the other membranes must have been moulted within this same period at least thirty times. In a young specimen of this species, having the orifice of the shell only two tenths of an inch in diameter, I found the opercular membrane, as usual, double. This membrane is not furnished with spines, nor is it penetrated by tubuli as in most other genera. The tissue left after the opercular valves have been dissolved in acid, presents no tubuli, or any other recognisable character, and is not divided (as is usual) into layers.

_Muscles of the Sack._--These muscles differ considerably from those of other sessile cirripedes hitherto described. There are the usual five (or properly six, the two tergal muscles being here, as elsewhere, confluent) bundles of fasciæ; but they hardly can be called bundles, they are so much spread out and thin. In _C. balænaris_, each rostral muscle consists, in different specimens, of from three to five principal fasciæ; these, at their upper ends, are attached to the cartilaginous plicæ at the rostral extremities of the scuta; at their lower ends, they do not reach (as in all the previous genera) to the basal membrane, but after converging, they diverge again into a little fan of fibres, which are firmly attached to the corium low down on the sides of the sack. The lateral depressores of the scuta consist each of about three fasciæ, and they terminate downwards like the rostral muscles. The tergal depressores are spread out into a thin sheet; upwards they reach to the basal edges of the lips of the sack-aperture, and downwards they curve a little towards the rostral and opposite end of the shell, and extend nearly to the basal membrane. These tergal muscles include two fasciæ, larger than the others, which extend rather further, both upwards and downwards, than the other fasciæ. But the most novel character in these several muscles is that in their lower portions they do not exhibit transverse striæ, thus showing a tendency to become involuntary as in pedunculated cirripedes. This circumstance, and their feebleness, is easily accounted for by the thick unyielding nature of the opercular membrane, and the feebly developed character of the opercular valves. In _C. diadema_, the tergal muscles are much spread out, having, as in _C. balænaris_, a larger fascia on each side; but the lateral depressores of the scuta form a well defined nearly cylindrical bundle; the rostral pair are extremely weak and spread out: I could perceive only feeble transverse striæ on some of these muscles, and on others there was not the least trace of striæ. I may add that the adductor scutorum muscle is well developed, as are the eight pairs of muscles which unite the animal's body to the under surfaces of the scuta. The action of the adductor scutorum serves to close the sack-aperture, but towards the carinal end necessarily with very small force; protection in this part can only be afforded by the protuberant, valvular lips, and by the dorsal surfaces of the inwardly curled cirri, with their tufts of bristles.

_Mouth._--The mouth (Pl. 26, figs. 3, 4) is much elongated transversely, and does not differ essentially from that of most other Balanidæ. The labrum is notched and not in the least bullate, though in _C. diadema_ there is a slight prominence outside close beneath the notch. The palpi are of large size, with their bristly apices touching each other. The mandibles are very strong, with from four to five main teeth, which are remarkable by presenting only rudiments of being laterally double; but between the second and third, and between the third and fourth main teeth, there is a small intermediate tooth,--these I have not met with in any sessile cirripede hitherto described. The maxillæ are small, with the two upper spines remarkably strong. The outer maxillæ are on their inner faces bilobed. Between these organs there is a minute projection, or mentum, flattened in the longitudinal axis of the body; I have not noticed this in any previous Cirripede.

The _Cirri_ are short and extremely much flattened: the three anterior pairs have their rami unequal in length by two or three segments; the posterior edges of their pedicels are fringed by tufts of extremely fine hairs. The pedicel of the first cirrus is very broad; its rami are short, with the segments very broad. The rami of the second and third cirri are short, with the segments protuberant in front and thickly clothed with spines; the terminal segments have some short, thick, claw-like spines. The three posterior pairs have protuberant segments, each supporting three or four pairs of short, strong, main spines, with a small intermediate tuft: the dorsal tuft is small.

The prosoma is of large size. The _stomach_ is large, without cæca, but with some internal longitudinal plaits (in _C. balænaris_ at least), showing a tendency to the formation of cæca. In the stomach of _C. balænaris_ I found a considerable quantity of a conferva, too much, I think, to have got in accidentally.

_Generative system._--The vesiculæ seminales are large; and at their broad blunt ends, in _C. balænaris_, four separate vasa deferentia enter, of which fact I have seen no other instance. The ovarian tubes do not extend up the sides of the sack, but lie at the bottom, over the basal membrane; in _C. balænaris_ they send six short ray-like prolongations into the six sutures; in _C. diadema_ they send similar prolongations into the sutures, and fill up, as I believe, the six chambers (Pl. 16, fig. 7, _v_) lying between the radii and alæ: I have examined only one specimen of _C. diadema_ in spirits, and this had the ovarian tubes in an early state of development, when they can with difficulty be distinguished from the pulpy corium; the orange-coloured masses, however, which filled the six cavities, resembled the layer which certainly consisted of undeveloped ovarian tubes and cæca, lying over the basal membrane. The ova are wonderfully numerous; their length is 3/400ths of an inch. The larvæ have been noticed in the anatomical introduction.

_Branchiæ._--These are immensely developed, covering almost four fifths of the area of each side of the sack. Each consists of two nearly equal folds, attached vertically to the carinal end of the sack, and transversely across the upper end extending to the animal's body. The outline of the free part is rounded. Both folds of both branchiæ are deeply plicated on both sides; hence the superficies of the whole is very great. We shall find that this structure is common to the three following allied genera, but with these exceptions, I have observed double branchiæ only in one species of one other genus, namely, in _Chthamalus dentatus_.

_Attachment._--The shells adhere with remarkable strength to the whale's skin. Having, until recently, examined only separated specimens, and observing portions of the whale's skin adhering to the outside, and solidly filling up the cavities on the under side, I did not doubt that the shell had the power of forming, by its own action, a deep cavity in the skin of the whale. Inspection of the basal outline of the walls of the shell (Pl. 16, fig. 5) of _C. balænaris_, will show how singularly unfitted its structure is for any burrowing process; and I was led to speculate on the possibility of the pupa being able to bury itself deeply in the skin, but rejected this view as opposed to what is known of the habits and structure of the pupæ of other Cirripedes. Having now examined several specimens of _C. diadema_ adhering, in a group, to a large piece of skin, in Mr. Stutchbury's collection, it has become evident that the attachment is as much owing to the upward growth of the whale's skin, as to the downward growth of the Coronula. In Pl. 15, fig. 4, a vertical section is given of the whale's skin, taken through the place whence a shell of _C. diadema_ has been removed; consequently we here see nothing but the whale's skin: the upper black layer is the dark horn-coloured epidermis, forming the general surface of the whale's body, and resting on an orange-coloured fibrous layer, which is lightly shaded in the drawing. The two horns in the section are two of the eighteen projections, formed entirely of the dark epidermic layer, which fill up the eighteen flattened cavities produced by the folding of the walls. Outside the horns we see the section of a circular furrow, in which the circumferential margin of the shell was lodged; and between the horns, there is the central hollow, within which, when lined with shell, the cirripede's body was included. The circular furrow is formed in main part by the epidermic layer being thinner there than on either side, and partly by the orange-coloured, underlying fibrous layer curving a little downwards, from having apparently yielded to the pressure of the circumferential margin of the shell. With respect to the cause of the thinness of the epidermis under the circular furrow, I do not know how much to attribute to mere mechanical compression or stretching, and how much to the pressure of the shell, having checked[124] its formation. In the case of very young and small shells, it is hardly possible that their pressure can have in any way influenced the formation of new epidermic layers under the thick old layers; and we must believe, at least in these cases, that the whole effect is mechanical, the sharp basal edges of the shell having indented the epidermis; but this is not more surprising than that the radicle of a plant should penetrate hard ground. Whether the indented epidermis in the circular furrow becomes ruptured, I am not sure; ragged layers may commonly be observed outside the shell, but it is very possible that these may be the ends of layers of epidermis which have been preserved by the covering of the shell, whereas, on the surrounding parts of the whale's body, these same layers have been removed by disintegration. To return to the section,--the outline of the boss of orange-coloured fibrous tissue, under the central hollow, clearly shows that it must have been formed by its own upward growth, for it stands above the general surrounding level of the corresponding layer. This same conclusion is still more obvious with respect to the eighteen flattened prominent horns, formed of the dark epidermis; the manner in which the epidermis has been forced, moulded, and packed into the eighteen flattened and curved cavities of the shell, so as to adhere to them with considerable tenacity, is extremely curious. The prominence of these horns is so great that it appears to me quite impossible to account for them, excepting by a special formation of epidermis beneath each cavity. The basal membrane of the Coronula, which lies at the bottom of the central hollow, adheres by its own cementing apparatus; and when the larva first attaches itself, this adhesion must be very important, as it allows the basal edges of the shell, during their slow downward growth, to press firmly on the whale's skin, and so slowly indent it with the circular furrow. The final cause, probably, of the cavities on the under side of the shell in this genus, formed by the singularly convoluted parietes, is to allow of the upward growth into them of the epidermis of the whale, thus securing a firm attachment and allowing the shell to exert a strong downward pressure, and thus effect its partial imbedment, and protection from the enormous force of the waves to which it must be exposed.

[124] Formerly I was inclined to believe, as stated in my former
volume on the Lepadidæ, that the cement injured the true skin of
the supporting animal, but this, at least in such cases as the
present, I do not now at all believe.

With respect to _C. balænaris_, I have seen only specimens, preserved on shrunk and twisted whale's skin, with the underlying fibrous layer not preserved; but the cavities in the shell were filled by horns of epidermis, exactly as in _C. diadema_. There is, however, this difference in the attachment of the two species, that in _C. balænaris_, owing to its depressed form, the circumference of the shell indents the whale's skin, not vertically downwards, as in _C. diadema_, but very obliquely outwards; and, consequently, buries itself much more completely, but less deeply, under a folded and apparently ruptured flap of the epidermis. In young specimens, of the size of a shilling, the entire shell, with the exception of the operculum, is thus covered up and protected whilst young and tender.

_Geographical Distribution._--The genus is found wherever whales occur, that is, from the Arctic to the Equatorial regions, in both hemispheres. It is asserted that sometimes as many as a couple of hundred specimens will adhere to a single whale. _Coronula barbara_, a form closely allied to _C. diadema_, existed during the Red Crag period; and Bronn has described some fossil specimens from Italy.

_Affinities._--In the wonderfully convoluted shell,--in the parietal tubes not being either filled up by calcareous layers or being crossed by calcareous septa,--in the outer lamina of the shell between the longitudinal septa near the basis being imperfect, Coronula differs from all the foregoing genera; in the two latter respects it agrees with the three following allied genera, viz., Platylepas, Tubicinella, and Xenobalanus. The equal size of all six compartments of the shell, has been observed in very few genera besides Coronula. In a new opercular membrane not being formed at each exuviation, and in two or three of these membranes being persistent, and in their being attached high up the sheath, this genus agrees with the three following genera alone. In the valves tending to be rudimentary, and in the protuberant lips of the sack-aperture, we have a close alliance with Xenobalanus. The muscles of the sack being spread out, and tending to lose their transversely striated character, are great peculiarities in Coronula, Tubicinella, and Xenobalanus. The simplicity of the cement-ducts is a remarkable character, observed in two of the following genera, but not in Tubicinella. The double branchiæ is a peculiarity common to all four genera. Neither the mouth nor cirri offer any new characters of much importance in Coronula or in the three allied genera: the lower teeth of the mandibles not being laterally double, but having small intermediate teeth, is the newest feature in the mouth.

1. CORONULA BALÆNARIS. Pl. 15, fig. 2, 2 _b_: Pl. 16, fig. 3, 5.

LEPAS BALÆNARIS. _Gmelin._ Systema Naturæ (1789).

---- ---- _Chemnitz._ Conch., vol. 8, Tab. 99, fig. 845, 846 (1785).

BALANUS ---- _Bruguière._ Encyclop. Method., Pl. 164, fig. 13-18
(1789).

CORONULA ---- _Lamarck._ Annales du Museum, vol. 1, Tab. 30, fig. 2-4
(1802).

---- ---- _Chenu._ Illust. Conch., Plate, fig. 1 and 4.

---- ---- _De Blainville._ Dict. des Scien. Nat., 1818 and 1824,
Tab. 117, fig. 3, 3 _a_.

_Shell much depressed, with longitudinal flattened ribs, having simple edges; orifice rounded-hexagonal: radii very thick, almost equalling the shell in thickness: opercular valves four._

_Hab._--Attached to whales in the Southern Ocean.

Having described, under the genus, in so much detail the structure of the whole shell, it will be sufficient here to point out the characters by which this species differs from the others.

_General Appearance._--The shell is generally much depressed, though
sometimes, in large specimens, from the turning in of the basal
edges, a considerable degree of convexity is acquired. The radii
are moderately wide, and give a star-like appearance to the shell.
The surface of the whole upper part of the shell is smooth; the
broad and much flattened ribs (_i. e._ the terminal, transverse
loops of the folded walls) of which each compartment is formed,
generally divide at a short distance from the apex. The close sutures
separating these ribs are straight, and not finely serrated, by which
character alone this species can at once be discriminated from the
others. The surface, when closely examined, is found to be finely
striated longitudinally, and is transversely crossed, chiefly in
the lower part, by minutely beaded growth-ridges. The orifice is
rounded-hexagonal, and is small compared with the whole shell; it is
also smaller than the basal edge of the internal cavity of the shell;
consequently the whole basal edge cannot be seen through the orifice
from one point of view,--the operculum of course having been removed.
An unusually large specimen was 2.75 of an inch in diameter; this
individual was one of the more convex varieties, and yet its entire
thickness, from top to base, was only .9 of an inch.

_Operculum._--Having described, under the genus, the general
structure of the opercular valve, I will here only add a few details
on their shape. They are small compared with the whole extent of
the opercular membrane. The scuta (Pl. 15, fig. 2 _b_, scutum to
the left) stand near each other at the rostral end of the aperture
leading into the sack, with their two rostral ends united by the
yellowish-brown, longitudinally plicated, horny membrane, described
under the genus. The terga (fig. 2 _b_) stand a little apart from
the scuta, on the sides of nearly the middle of the sack-aperture.
The lips are protuberant and moderately developed all round. The
scuta are elongated, and a little curved; including the upper
imbedded portion, they are almost sub-triangular: but the under
growing surface is much elongated, nearly flat, with the two ends
of nearly the same width, truncated and rounded. The _Tergum_, in
rather small specimens is, if the upper imbedded portion be included,
sub-triangular, with the growing surface oval, and between one third
and one half of the length of the scutum; but in large specimens,
the tergum becomes style-formed, lying parallel to the tergal margin
of the scutum, with the growing surface proportionally much smaller,
and not above one fifth of the length of the growing surface of
the scutum. Hence we see some tendency in the tergum to become
rudimentary, as it is in the next species. The brown, horny, plicated
substance in which the terga are imbedded, extends considerably
beyond the valves themselves.

_Structure of the Shell and Radii._--I have already so fully
discussed this subject, that I will here only enumerate the points in
which this species differs from _C. diadema_ and _reginæ_:--Firstly,
the more symmetrically folded walls (Pl. 16, fig. 5), new folds
arising on both sides of all six sutures. Secondly, the inner ends of
the folded walls, which surround the internal cavity, being almost
square, but with their angles rounded; their inner ends descend some
little way beneath the basal edge of the sheath, as low, or lower,
than the circumference of the shell. Thirdly, the external lines of
junction between the transverse terminal loops being smooth or not
serrated. Fourthly, these loops being flatter towards the outside,
and being internally filled by septa (Pl. 15, fig. 2 _a_); tubes
being thus formed larger than the proper parietal pores. Sixthly, the
sutural edges of the compound radii (Pl. 16, fig. 3) being very much
broader in the inner and upper part of each compartment than in the
outer and lower part; for in the inner and upper part they stretch
from the outside of the shell to the sheath, so that the alæ rest on
them, and hence no large open cavity is left between the radii and
alæ; in the inner and upper part, also, the radii extend down nearly
to the basal edges of the folded walls. The septa, of which the radii
are formed, stand further apart than in _C. diadema_ and _reginæ_.
Seventhly, the alæ are only 1/5th or 1/6th of the thickness of the
radii, whereas in the two other species they are very much thicker,
being nearly as thick as the radii: the alæ are also here (fig. 3,
_a′_) squarer than in those two species, that is, their basal margins
are not so short compared with their upper margins; their edges
present, also, a slightly different structure. Eighthly, the basal
edge of the sheath does not here project freely, and does not descend
quite so close to the basal membrane.

_Mouth._--The palpi are furnished with a row, extending along the
whole basal exterior margin, of very long upwardly pointing spines;
on their upper edges there is a brush of small spines. The edge of
the labrum is clothed with extremely fine spines, and is furnished
with a few minute teeth. The mandibles have four teeth, and the lower
end is broadly rounded and coarsely spinose: the second and third
teeth are double at their points, and between the second and third,
and again between the third and fourth, there is a small intermediate
tooth.

Regarding the _Cirri_, I have nothing especial to remark.

_Geographical Distribution._--I have examined nine sets of specimens,
having localities attached to them: three from off New South Wales;
two off the Cape of Good Hope; two from the west coast of South
America; and two marked only "South Sea." Hence I am led to conclude
that this species is confined to the southern hemisphere, or if it
extends into the northern hemisphere, it is probably only in the
Pacific Ocean. I do not believe (though so stated in some works) that
this species occurs on the shores of Europe. It seems often to be
associated with Tubicinella. Some specimens thus associated, sent by
Mr. Bennett to Professor Owen, were said to have been attached to the
_Balæna australis_.

2. CORONULA DIADEMA. Pl. 15, fig. 3, 3 _a_, 3 _b_; Pl. 16, fig. 1, 2, 7.

LEPAS DIADEMA. _Linn._ Systema Naturæ, 1767.

---- ---- _Chemnitz._ Conch., vol. 8, Tab. 99, figs. 843, 844.

BALANUS DIADEMA. _Bruguière._ Encyclop. Method., n. 164, fig. 13, 14
(1789).

CORONULA ---- _De Blainville._ Dict. des Sc. Nat. (1824), Tab. 117,
fig. 4.

---- ---- _Leach._ Encyclop. Brit. Suppl., vol. iii, 1824.

---- ---- _Chenu._ Illust. Conch., Plate, fig. 3.

---- ---- _Burmeister._ Beiträge zur Naturgeschichte der
Rankenfüsser, 1834, Tab. 2, fig. 1-14, 18.

_Shell crown-shaped, with longitudinal convex ribs, having their edges crenated; orifice hexagonal: radii moderately thick, very broad: terga absent or rudimentary._

_Hab._--Attached to whales, in the Arctic Seas; United States and
Great Britain; Gulf-Stream, Atlantic Ocean; New Zealand(?).

_General Appearance._--As previously remarked, owing to the fulness
of the generic description, minute details on structure, excepting
those characteristic of the present species, need not here be given.
The crown-like shape of the shell is well expressed by its name of
_Diadema_, but the crown tends to pass into a cylinder. The radii are
extremely broad. The orifice is large, and neatly hexagonal: when
the operculum is removed the whole inside of the cup-formed shell
can be seen at once, for the flat membranous basis is much smaller
than the orifice. The under side of the shell is deeply concave.
The outside of each compartment is formed by broad, rounded, and
somewhat prominent, rarely divided, ribs (_i. e._ the transverse
ends of the folded walls); these ribs are closely united together
by finely serrated lines of junction (Pl. 16, fig. 1, _f_). Their
surfaces outside are delicately striated longitudinally, and plainly
crossed (more plainly than in the foregoing and the next species) by
irregular, transverse ridges, especially in the lower part of the
shell. The largest specimen which I have seen was two and a half
inches in diameter and two in height.

_Scuta._--These are placed close together at the rostral end of the
orifice, and are imbedded in the brownish, tough, longitudinally
plicated, horny substance, which extends far beyond both ends of the
valves. In outline (fig. 3 _b_) they are mitre-shaped, or rounded
and sub-triangular, a little curved, and more or less elongated,
being most so in young specimens; they are, however, less elongated
and rather more massive than in _C. balænaris_. _Terga_,--these seem
entirely absent in most specimens; but in one (fig. 3 _a_) I found
a rudiment, namely, a short thin plate of shell, barely visible to
the naked eye, extending parallel and near to the tergal margin of
the scutum. The lips of the aperture of the sack are prominent, and
highly so towards the carinal end.

_Structure of Shell and Radii._--Owing to the shell not spreading
much at the base, new folds in the walls are much seldomer formed,
and therefore the external longitudinal ribs (_i. e._ the terminal
transversely elongated loops), are much seldomer divided, than in _C.
balænaris_ or _reginæ_; even rather large specimens sometimes having
only the original eighteen folds.[125] When new folds are formed,
only one is formed on one, viz., the rostral, side of each suture,
instead of on both sides, as in _C. balænaris_. The inner ends of the
folded walls, surrounding the basal membrane, are narrow, instead
of being almost square, as in _C. balænaris_. The lower edge of the
sheath, which projects freely, descends almost to the level of the
basal edges of the walls. The outer ends of the folded walls, forming
the transverse loops, are internally filled up solidly by calcareous
matter, instead of by septa forming tubes, as in _C. balænaris_.
The radii (Pl. 16, fig. 2) are a little thicker in the lower and
outer than in the upper and inner part of each compartment; in the
middle, they do not reach the sheath by about half the thickness of
the compartment, and consequently they are separated from the plates
(_c′′_ in fig. 1 and 7) on which the alæ rest, by large chambers (_v_
in fig. 7), which extend up to nearly the apices of the compartments:
the extent, however, to which the upper ends of these chambers have
been solidly filled up, varies a little. The sinuous plates forming
the main portion of the compound radii are rather thinner and closer
together than in _C. balænaris_. The alæ are thick, being thickest in
the middle part, and there equal the radii in thickness; their lower
margins are very short compared with their upper margins, hence they
are almost wedge-formed.

[125] These should be counted on the under side of the shell,
for if counted from the outside, the number would be thought to
be twenty-four, as on the side of each compartment bordering the
radius, a half-fold has the appearance of being a whole fold, so
that in appearance six folds are added to the real number of the
folds. This caution is necessary whatever the number of real folds
may be, that is, whether or not the original eighteen folds have
been increased by subsequent plications. These remarks, also, are
applicable to the other species.

_Mouth._--The teeth and fine hairs on the labrum are sometimes
obscure, and sometimes plain: close outside the bottom of the medial
notch, there is a small hard prominence. The palpi are broad; on
their basal exterior margins there is a short row of spines, which
do not equal in length the width of the palpi, and therefore are not
so long as in _C. balænaris_. The mandibles have five main teeth, of
which the second and third show only an obscure rudiment of being
double; between these two teeth, and between the third and fourth
tooth, there is a small intermediate tooth: the inferior angle is
narrow, rounded, and spinose. The extremity of the apodeme of the
maxillæ is expanded.

_Cirri._--I have only to remark, that the pedicel of the first cirrus
is extremely broad, and that the rami are set on in an unusually
crooked manner: the basal segment of the shorter and broader ramus of
this cirrus has its dorsal surface produced into a plate fringed with
very fine hairs.

_Geographical Distribution._--I have received only four specimens
with certain localities attached to them: namely, the Arctic Seas of
Scandinavia; the coast of the United States, and of Britain; and the
Gulf-Stream. There is also a specimen in the British Museum, sent by
Mr. Stephenson, mingled with shells of Mollusca from New Zealand;
but a Coronula, procured from a floating whale in the early part
of the outward voyage, might so easily be sent home with specimens
subsequently collected in another county, that I do not as yet fully
admit that this species is an inhabitant of the Southern Pacific
Ocean: I am less willing to admit this, from suspecting that _C.
reginæ_ in the Pacific, replaces the _C. diadema_ of our Northern
Seas.

3. CORONULA REGINÆ. Pl. 15, fig. 5; Pl. 16, fig. 4.

_Shell globulo-conical or depressed, with longitudinal, much flattened ribs, having their edges crenated, and their surfaces striated and granulated; orifice hexagonal: radii thin, not exceeding one fifth of the thickness of a compartment: terga absent._

_Hab._--Attached to whales, Pacific Ocean; Mus. Cuming, Stutchbury,
and Darwin.

I have seen eight specimens of this form--two in Mr. Cuming's collection, one of which was procured at Iquique in Peru, the other from an unknown locality; one was given me in Chile, and was stated to have come from the Pacific; two others are in Mr. Stutchbury's collection, from unknown but distinct localities, and three in a group in the British Museum. These eight specimens, of which six were collected separately, all strikingly agree in general aspect and in details of structure, so that I can easily recognise the shell, and can at once pick out a single compartment, when mingled with those of the two foregoing species; nevertheless, the differences are so small, that I at first hesitated whether to name the species; but, upon reflection, I am convinced that it is distinct. _Coronula reginæ_ is much more closely related to _C. diadema_ than to _C. balænaris_, and I suspect that it replaces in the Pacific the former of these two species.

_General Appearance_; shell conical, straight-sided; some specimens
being nearly as much depressed as _C. balænaris_, and considerably
more depressed than any variety of _C. diadema_; other specimens
being globulo-conical, but rather less globular than the ordinary
form of _C. diadema_. I may here remark, that shape is of more value
in this genus, in which the shell is attached to the yielding skin
of whales, than in those genera in which it adheres to rocks. The
orifice is neatly hexagonal, and the whole internal cavity, when the
opercular membrane is removed, can be seen from one point of view,
owing to the contraction of the lower part of the cavity and small
size of the basal membrane. The surface of the shell is smooth, but
has, even up to the summit, a peculiar frosted appearance, different
from that of the foregoing species, caused by the longitudinal
striæ being rather more distinct, and being crossed by beaded, very
delicate transverse lines of growth. The ends of the transverse
loops, forming the exterior surfaces of the compartments, are much
flattened, even more so than in _C. balænaris_. The lines of junction
between the loops, are finely serrated, as in _C. diadema_; and
internally they are solidly filled up, instead of being formed into a
set of tubes by longitudinal septa, as in _C. balænaris_. The under
side of the shell, with its folded walls, presents an appearance
intermediate between the variable appearance of this part in the
above two species.

The sutural edges of the radii (Pl. 16, fig. 4) offer by far the
most remarkable character, in their thinness from top to bottom; for
they hardly exceed one fifth of the thickness of the compartment,
measured from the external surface to the base of the sheath; hence
a very large cavity is left between the radii and alæ: in the
thickness of the radii the three species already described form a
series, _C. reginæ_ at one end and _C. balænaris_ at the other. The
sinuous plates, forming the lower part of the radius, are coarser and
stand rather further apart than in _C. diadema_. The alæ are thick,
and have the same outline, being narrow at their basal margins and
broad at top, as in _C. diadema_, with their sutural edges similarly
constructed: the basal edge of the sheath likewise projects freely.

_Operculum._--This resembles most closely that of _C. diadema_. There
are no rudiments of terga. The scuta cannot be distinguished from
those of _C. diadema_.

_Mouth._--The labrum has a row of inwardly curved little teeth along
the whole crest, and these I did not notice in _C. diadema_: there
is only a trace of the prominence on the outside at the bottom of
the central notch. The hairs on the basal exterior margin of the
palpi are moderately long. The mandibles have five teeth. In the
_Cirri_, the first pair resembles in its peculiar structure those of
_C. diadema_. In the sixth pair, however, the segments support only
three pairs of main spines: but the specimen was not very large, and
probably in old specimens there would have been four pairs.

_Summary._--The present species differs from both the foregoing
only in its rather more conical and straight-sided outline, smooth,
frosted surface, and in the narrowness of the sutural edges of the
radii, and consequent large size of the chambers between the radii
and alæ. It resembles _C. diadema_, as far as the shell is concerned,
in the external ribs or transverse loops having their lines of
junction serrated and in being solidly filled up--in the shape of
the orifice and of the internal cavity of the shell--in the shape
and structure of the alæ--and in the basal edge of the sheath being
free. It comes nearer to _C. diadema_ than to _C. balænaris_ in the
structure of the radii. It differs from _C. diadema_, and comes
nearer to _C. balænaris_, in the external ribs being flattened,
instead of being convex, and in the lines of growth being very
delicate. But as it resembles _C. diadema_ in the several foregoing
characters of its shell, in the opercular valves, in all parts of the
mouth (excepting the labrum), and in the cirri, it is very much more
nearly related to that species than to _C. balænaris_.

4. CORONULA BARBARA. Pl. 15, fig. 6.

CORONULITES DIADEMA (?) _Parkinson_, Organic Remains (1811), vol. 3,
p. 240, Pl. 16, fig. 19.

_Shell (probably) crown-shaped, with longitudinal convex ribs, having their edges crenated, and their surfaces rugged, both externally and internally, with transverse ridges: radii moderately thick; the spaces between the radii and the alæ solidly filled up._

_Fossil_ in Red Crag, (Bawdsey and Sutton); Mus. S. Wood and
Geological Society.

The species here described, though near to _C. diadema_ and easily confounded with it, I have no doubt is distinct. I owe to the kindness of the Rev. Mr. Image an examination of the original specimen figured by Parkinson; and in Mr. Stutchbury's collection there is a similar and more perfect specimen; both of these resemble _C. diadema_ in general form, but have been too much worn to be positively identified. The following description is drawn up from some compartments collected by Mr. Searles Wood, belonging certainly to three, and probably to four individuals, one of which was young; as these specimens agree in all essential respects, I feel pretty confident that the characters, by which the present species differs from _C. diadema_, are of specific value.

_Structure of Shell._--The longitudinal ribs on each compartment
(_i. e._ the terminal transverse loops), are convex and prominent,
as in _C. diadema_, but they are crossed by more prominent ridges
of growth than even in the roughest varieties of that species, so
that the surface of the shell is more rugged. In the three previous
species, the surface of the wall entirely round the cavities occupied
by the whale's skin, is striated only by longitudinal very fine
lines; but here, the outer portion, or that (fig. 6) formed by each
transverse loop, is crossed by transverse ridges of growth, like,
but less prominent than, those on the external surface of the shell.
The minute teeth, along the lines of junction between the transverse
loops, are here less regular, and can hardly be said to exist; for
the two edges are locked together by what may be more strictly
called minute zig-zag ridges than teeth. The exact number of the
circumferential plications in the walls is variable in the same
manner as in the three foregoing species. The sutural edges of the
radii are about as thick as, or rather thicker than, in _C. diadema_.
As in this latter species, and in _C. reginæ_, each ala rests, not
on the internal surface (as in _C. balænaris_, and in all other
Balanidæ) of the radius, but on a special plate; but in _C. barbara_,
instead of a deep chamber, running up to the apex of the compartment,
being left between the radius and ala, this part is filled up almost
entirely by solid shell. Although the extent to which this chamber is
filled up varies a little, and although its depth varies a little in
_C. diadema_, yet there is a marked difference between the specimens
of this latter species, in which the chamber is most filled up, and
those of _C. barbara_, in which it is least filled up. The alæ are
thick, as in _C. diadema_, and their sutural edges have a central
ridge, sending off on both sides sinuous ridges. The basal margins
of the alæ are not short compared with their upper margins, and
therefore the whole ala is not wedge-formed; and in this rather
important respect _C. barbara_ resembles _C. balænaris_ (Pl. 16,
fig. 3), and differs from _C. diadema_ (fig. 2). The lower edge of
the sheath does not seem to have projected freely,--in this respect,
also, resembling _C. balænaris_. From the basal margin of the alæ not
being narrow, and from the inner ends of the folded walls being, as
it would appear, also broad, I have little doubt that the cavity in
which the animal's body was lodged, resembled in shape that in _C.
balænaris_, the membranous basis being larger than the orifice of the
shell.

Opercular valves unknown.

_Summary._--This species is more nearly related to _C. diadema_ than
to the others; but in some points, just specified, it resembles _C.
balænaris_. The characters by which it differs from all the species
are, firstly, the more prominent transverse ridges on the external
surface of the shell, and more especially on the surfaces bounding
the outer sides of the cavities occupied by the epidermis of the
whale. Secondly, the character of the teeth, or rather ridges, along
the lines of junction between the transverse loops. And, thirdly, the
spaces between the radii and the plates on which the alæ rest, being
solidly filled up.

_Species Dubiæ._

The _Coronula bifida_ is an Italian tertiary species, so named by Bronn, in his "Italiens Tertiär-Gebilde" (1831), p. 126. It is very possible that this may be identical with _C. barbara_, but Bronn does not seem to have been aware of the absolute necessity of giving minute details in his descriptions of fossil cirripedes. The chief character of _C. bifida_ is thus given:--"Eine tiefe Furche oder Spalte theilt die Längenrippe von oben herab bis zur Hälfte, welche bei der sonst ähnlichen _C. diadema_ entweder ganz fehlt, oder nur zuweilen kurz angedeutet ist." Had it been stated that the longitudinal ribs were divided from the middle down to the base, instead of from the top to the middle, the description would have been intelligible to me, though the character thus afforded would not have been of specific value, as this dividing of the ribs occasionally occurs in all four species, and is produced by the formation of new folds to the walls.

9. _Genus_--PLATYLEPAS. Pl. 17, fig. 1 _a_-2 _b_.

PLATYLEPAS. _J. E. Gray._ Annals of Philosophy, (new series), vol. x
(1825).

CORONULA. _De Blainville._ Dict. des Sciences Nat. (1824).

COLUMELLINA. _Bivona_ (fide _Philippi_). Nuovi generi, &c. di
Molluschi, Palermo (1832).

_Compartments six, each bilobed and inwardly produced, so as to form six midribs, which support the outwardly convex, membranous basis._

_Distribution_, throughout the tropical and warmer temperate seas.
Imbedded in turtles, sea-snakes and manatee.

This small genus, consisting of three species (though I have named only two), is a very natural one, yet closely allied to Coronula. Most authors have united these genera, but in doing so they destroy two very natural little groups. Platylepas, moreover, when all its characters are considered, has as good a claim to be generically separated from Coronula, as has Tubicinella,--a genus universally admitted. I shall presently recur to this subject.

_General Appearance._--The most remarkable character, and which gives a peculiar aspect to the whole shell, is that each of the six compartments has a medial fissure or fold; the shell thus consisting of twelve lobes. Each fold extends from the summit to the base of the shell, and is produced inwards in the shape of a midrib (Pl. 17, fig. 1 _a_). The membranous basis is supported by the basal edges of the six midribs, and is thus rendered convex. As the midrib on the rostrum would have interfered, if fully developed, with that part of the animal's thorax which I have called the prosoma, it is of less size than the other midribs; and from this same cause the whole rostrum is pushed a little on one side, generally to the left, so that the shell is not perfectly symmetrical. The midrib of the carina is less than the lateral midribs. The orifice is oval, and is more or less plainly indented by six angular points,--the worn-down summits of the six midribs. The narrow elongated opercular valves stretch from one end of the orifice to the other. The rostrum is much broader than the carina: the lateral compartments are but little broader than the carino-lateral compartments. The radii are narrow, though variable in width. The general shape of the shell varies much in the same species, being either much depressed, or rather steeply conical; either circular, or more commonly oval. The (so called) epidermis is usually persistent in the lower part of the shell. The shell itself is white. The basal diameter of the largest specimen of _P. bissexlobata_ was nearly three-quarters of an inch.

_Structure of the Parietes, Radii, &c._--The walls of the shell in _P. bissexlobata_ are permeated by minute pores, and have exactly the same structure as in Coronula; the pores being completed by the union of ledges on the outer sides of the longitudinal septa, and the latter have little knobs formed on them at each period of growth. In _P. decorata_ the walls are solid. Each compartment, as already stated, has a deep nearly medial furrow, the sides of which are closely pressed together, thus forming the midribs, and thus differing from the furrows in Coronula, which not being pressed together, form cavities on the under side of the shell. The inner longitudinal margin of each midrib is thickly coated by a layer of shell, and is thus rendered much more prominent than it would have been if formed exclusively by the inward folding of the wall. The upper part of each midrib forms a slight, longitudinal ridge (fig. 1 _d_) on the sheath, having become encased by the sheath during its downward growth. Owing to the small size of the midrib of the rostrum, there is scarcely any ridge on the sheath of this compartment. The sheath does not descend half-way down the walls in _P. bissexlobata_, but further down in _P. decorata_; but perhaps the thickened inner margins of the midribs may be considered as the downward prolongations of the sheath, and on this view the sheath descends actually to the basal membrane. The radii are narrow: their edges have simple septa, with the interspaces filled up solidly. The alæ have their sutural edges smooth.

The _Basis_ is membranous, and more or less convex. This convexity is rendered possible by the support afforded by the basal edges of the six midribs. The so-called epidermis or membrane which covers the shell, is prolonged in six flaps under the midribs; and hence the proper basal membrane has six deep indentations, receiving the above flaps. The cementing apparatus is constructed on the same simple type as in Coronula, as has been described in the Introduction.