General Appearance.—Shell nearly circular, moderately or slightly depressed; colour gray. The eight normal compartments of the inner whorl are two or three times as large as those in the second whorl; the other smaller compartments or scales graduate very regularly in size, to mere beads, at the extreme basal edge, and are arranged symmetrically. The general aspect of the shell depends chiefly on the degree to which the surface has been disintegrated, and differs greatly in the two extreme states. When well preserved, the general appearance is very elegant; the scales are all neatly imbricated; they terminate upwards in points, but with their extreme tips generally broken; from laterally overlapping each other, their external surfaces (as seen, when corroded, in fig. 4 d, or when slightly exposed after the continued growth of their basal margins, in fig. 4 a) become longitudinally keeled, often with a secondary ridge or shoulder on one side; they are, also, crossed by rather conspicuous and regular lines of growth, or more strictly, former lines of union between the several valves: the orifice of the shell in this perfect condition is sub-rhomboidal and notched, and the scuta are united to the terga by nearly straight sutures. On the other hand, when the shell has been considerably corroded, and this seems to be the more common condition, the appearance is not elegant: the scales in the successive whorls are not imbricated, but owing to their upper parts having been worn down, they present a tesselated surface (4 d), with the tesseræ graduated in size, and of a peculiar shape, namely, a rectangle, with a more or less broad square projection on the exterior side, together often with a large square notch on one or both corners, caused by their laterally overlapping each other. Some of the shells are so deeply corroded, that no portion of the original surface is preserved, excepting the lowermost bead-like scales; and the whole shell has so rugged an aspect, that the successive whorls of the worn-down valves might easily be overlooked. In the corroded specimens, the orifice approaches to circular in outline, and is large and nearly entire: the scuta and terga are deeply interlocked together. The largest specimen which I have seen was one inch and a quarter in basal diameter.

Structure of the Shell.—The eight inner normal compartments are not thicker than the outer valves, and are far thinner than in ordinary sessile cirripedes. They are arranged as in Octomeris. The lateral compartments are broader than the rostro-lateral and carino-lateral compartments. The shelly laminæ, of which these compartments and the opercular valves are composed, alternate with yellow membranous layers, exactly as is the case with Octomeris. The lateral edges of the compartments, beneath the alæ, in the six compartments having alæ, bend inwards, especially just above the basis, so that the compartments in their lower parts (fig. 4 c) stand much more separate than is usual. The basal edge of each is irregularly toothed. The sheath presents no particular character. The eight compartments have longitudinal shoulders and ribs on their external surfaces, caused by the lateral overlapping and pressure of the exterior scales. They are also marked by slight, transverse or oblique calcareous ridges, caused by the attachment of the membrane, by which they are united to the smaller compartments or scales outside them. A new line of attachment, and consequently a new ridge is formed, lower and lower down at each period of growth, as the shell is added to at the basis,—in the same manner as new ridges are added to the lower edge of the sheath at each period of growth. The shell, excepting in old worn-down specimens, increases largely by diametric growth: during the diametric growth, the outer scales must be laterally separated a little from each other, and probably they are laterally added to; but there are no distinct lines of suture, or rows of smaller scales, corresponding with the sutures between the eight inner compartments. The alæ do not project much; their edges, as well as the shoulders into which they fit, are generally irregularly crenated: they are added to during diametric growth above the line of attachment of the opercular membrane. There is no appearance of radii; but as the eight inner compartments are added to laterally, and are often crenated, on the edges which correspond with the radii in other Cirripedes, such edges must be considered as radii.

With respect to the scales in the several outer whorls, they resemble each other except in size, and the outermost scales are reduced to mere transversely elongated beads. Their basal edges are concave inwards, being bent like the ridge of a house; hence sub-triangular spaces or tubes, lined by the corium, run up between the scales. When perfectly preserved, the outline of each scale is a much elongated triangle, but usually, from their summits having been worn off, the outline is nearly that of a parallelogram. Their basal edges are dentated, and their upper parts, both on the outside and inside, are marked, where joined to the other compartments, by slight calcareous ridges: outside, there are longitudinal shoulders (Pl. 20, fig. 4 b), caused by the lateral overlapping of the adjoining scales; these are best seen in section in corroded specimens (fig. 4 d): on the inside there are, also, in the upper part, slight medial longitudinal ridges, caused by the sutures, which the scales have covered.

In large old specimens there are ten, or even more, whorls of compartments, but it is scarcely possible to count them with any accuracy. The first whorl consists of the eight large inner compartments, though, homologically, it is doubtful whether the rostro-lateral compartments in any sessile cirripede really belong to the same whorl with the others. The second whorl consists of eight smaller pieces, covering the eight sutures in the first whorl (see the tracing of the basal edges of all the compartments and valves in a very perfect specimen, Pl. 20, fig. 4 c). The third whorl, in large and perfect specimens, consists of twice the number, or sixteen, still smaller scales, corresponding with the sixteen sutures of the second whorl; but sometimes there are less than sixteen pieces, owing to some of the scales being large enough to cover two adjoining sutures as well as the intermediate portion of the compartments of the first whorl. In the fourth whorl, instead of there being, even in the most perfect specimens, twice sixteen, or thirty-two pieces, there are only twenty-four; this being caused by single pieces (placed alternately with two pieces) being broad enough to cover two sutures as well as the intermediate portion of the compartment of the third whorl. In the succeeding whorls this same arrangement seems to be the usual one, so that in the fifth whorl, instead of there being, in the most perfect specimens, twice twenty-four, or forty-eight pieces, that is, twice the number in the last whorl, there are only thirty-six scales, or once and a half as many scales.

The Basis is thin and membranous; it firmly adheres to the surface of attachment.

Scuta, nearly flat: the articular ridge is very prominent, and there is a deep articular furrow both above and below; but the precise outline of the ridge and furrows varies: there is a pit for the adductor muscle, but no crests or marks for the other muscles. The Terga are remarkable from the extreme prominence of the articular ridge and depth of the articular furrow. The basal margin viewed internally seems straight, so that there appears to be no spur; but viewed externally, when the crests for the depressor muscles are seen to depend considerably beneath the true basal margin, a slight, very broad spur may be perceived to exist. These depending crests for the muscles are rather thin, but they extend over half the basal margin of the valve.

Mouth.—The labrum is very bullate, being as long in its longitudinal axis as the rest of the mouth: crest hairy, with some very minute teeth. Palpi truncated, with their apices not nearly touching each other; thickly clothed with spines. Mandibles with three large single teeth, of which the lower one has a single fine tooth at its upper basal edge, showing a tendency to become pectinated; inferior part short, coarsely pectinated. Maxillæ notched, with a slight second notch and slight double prominence in the lower part.

Cirri.—First and second pairs short, with the rami in each unequal in length by about four segments; on both rami in the second cirrus, and in the shorter ramus of the first cirrus, there are some coarsely pectinated spines. All four posterior cirri are alike; the segments bear five pairs of strong spines, with a large intermediate tuft of fine spines: the dorsal tuft is also large, consisting of short thick, and long finer spines. There is no vestige of caudal appendages, though present in the succeeding species.

Branchiæ moderately large, in area equalling the prosoma; surface not plicated.

As stated under the genus, this species is known from two specimens in the British Museum, one of which is full-sized, being three quarters of an inch in diameter, but is destitute of the outermost whorls, of the basis, opercular valves, and animal’s body; the other is perfect, but very young, being barely two tenths of an inch in basal diameter. As far as the characters can be made out from these materials, I can perceive no difference from C. polymerus in the shell, excepting that in the small specimen of C. imbricatus, there would appear to exist fewer whorls. The opercular valves are likewise closely similar: in the scuta, however, of the young specimen in the present species, the articular ridge seems to be a little broader, but this is so variable a character that no confidence can be placed in it: these valves, moreover, externally have a broad furrow along the middle, running from the apex to the basal margin, which is not the case with the scutum of C. polymerus; but then I have often seen, in young specimens of Balanus, a similar furrow, which is quite absent in full-grown specimens. When we come to the basis we find a good diagnostic character, for here it is calcareous: it is rather thin, solid, and white; towards the outside it is pitted with small cavities, corresponding with the small teeth on the basal edges of the compartments. The latter adhere firmly to the basis. The central internal surface is covered by an irregular network of imbedded cement-ducts, some of which bifurcate. From the description here given, it will be seen that any figure would have been superfluous, the last species having been so well illustrated.

Mouth.—I can point out no difference, excepting that the palpi are here more oval or less truncated at their ends; and that the lower corner of the maxillæ seems to be more prominent. In the Cirri, the rami of the first and second pairs are nearly equal in length: none of the spines are coarsely pectinated. In the four posterior pairs of cirri, instead of a tuft of small spines on each segment between the pairs of main spines, there are only a few minute intermediate spines: the dorsal tufts are also here smaller, but are, as in the last species, composed of short thick, and longer thinner spines.

Caudal Appendages.—This is the only species of sessile cirripede, with the exception of the two species of Pachylasma, which possesses these organs: they are situated on each side of the anus in the usual position: they are minute, equalling in length only the lower segment of the pedicel of the sixth cirrus: in a specimen in which the rami of the sixth cirrus had eighteen or twenty segments, these appendages consisted of only three tapering segments, supporting a few thick spines.

Branchiæ.—I believe I discovered these, consisting of two minute pouches, placed at the carinal end of the sack: if this observation be correct, this species differs from C. polymerus in the much smaller size of these organs.