Scuta.—These present no particular character, excepting in most cases the large development of the articular ridge, and sometimes the presence of a furrow above the articular ridge, of which only traces can rarely be detected in [Balanus]; hence in these cases, as in [C. Hembeli] (Pl. [18], fig. [5 a], [5 c]) and [scabrosus] (Pl. [19], fig. [2 a], [2 c]), the line of articulation between the scutum and tergum is more complicated than is usual. There is generally a slight pit, sometimes even furnished with small crests, for the lateral scutal depressor muscle: in [C. scabrosus] (fig. [2 d], q) a part of this muscle is attached to a small pit at the basi-scutal corner of the tergum,—a fact of which I have observed no instance in any other genus. In [C. intertextus] the terga and scuta are calcified together, without even a suture being visible on their internal faces. In [C. Hembeli] the valve is externally marked by a few longitudinal furrows.
The Terga, like the scuta, have a prominent articular ridge and deep furrow. In many specimens of [C. stellatus] and in [C. scabrosus] the valve is narrow: in [C. fissus] it is triangular and nearly equilateral. The crests for the depressores muscles are well developed: in [C. scabrosus] (Pl. [19], fig. [2 d], p) these crests are united into a plate, which, together with the outer lamina of the valve, forms a deep narrow pit: in [C. Hembeli] the crests are furnished with small sub-crests. The opercular membrane is narrow; it is sometimes furnished with a few minute spines.
Structure of Parietes.—The parietes are solid, and composed of successive layers of shell; the inner surface varies in condition in the same species, being either smooth or marked with branching impressed lines, or mamillated, or often irregularly punctured for the entrance of tubuli. Owing to these tubuli, the walls, when externally disintegrated, often become punctured. In certain depressed varieties of both [C. stellatus] and [scabrosus] the walls are supported by irregular depending columns, placed along either one side or both sides of the sutures. In [C. intertextus] we have the remarkable character of the wall of the shell growing (I presume after a certain age) almost rectangularly inwards, thus forming a rather wide, flat, calcareous rim round the central basal membrane. In [C. Hembeli] the internal basal edges of the parietes, in one moderately young specimen, were rugged with irregular points, but presented no other remarkable appearance; but in five old and very large specimens, the whole basis was calcareous, being absolutely continuous with the inner lamina of the parietes, showing that the latter had grown flatly inwards all round, and had then become confluent in the middle, so that there was no longer any basal membrane; excepting, no doubt, that which had existed in the younger stage, and which would be preserved in a functionless condition between the surface of attachment and the inflected parietes.
When an opercular valve or compartment is dissolved in acid, layers of tissue are left, and these are seen to be penetrated by tubuli, which enter at the punctures before mentioned on the inside of the shell: these tubuli often stand in groups of three or four together; they are about 1/10000th of an inch in diameter. Besides these irregularly scattered tubuli, there are in the opercular valves of [C. antennatus], innumerable smaller parallel tubuli, running to the external investing membrane.
Structure of the Radii and Alæ.—The radii, when developed, are always rather narrow. Their recipient furrows are generally nearly as broad as the radii themselves. Their edges are either quite smooth, as in [C. antennatus]; or very finely crenated; or, as in [C. dentatus] and [Hembeli] (Pl. [18], fig. [3 a], [5 a]), so strongly crenated as to make the suture, both externally and internally, toothed: in these two species, the radii are ribbed in transverse lines parallel to the basis, each rib corresponding with one of the projecting and interlocking teeth on the sutures. In [C. intertextus], and much less plainly in most specimens of [C. scabrosus] (Pl. [19], fig. [1 a], [2 a]), we have a structure in appearance very different, for the radii here consist of several very oblique plates, (i. e. nearly parallel to the parietes) on both sides of the sutures, which are interfolded or locked together: I believe that this structure is a mere modification of that in [C. dentatus] and [Hembeli], the transverse ridges on the radii of those species being here developed into oblique plates. We shall hereafter meet with a similar structure in the genus [Verruca]; to which genus, until meeting with these two species of [Chthamalus], I had thought that the interfolding sutures had been confined. The alæ have their edges generally finely crenated: during diametric growth (when such takes place), they are rarely added to above the level of the opercular membrane, and hence their summits are oblique: in [C. intertextus], however, the alæ are laterally added to above the opercular membrane, and their recipient furrows are likewise added to, of which fact I have seen no other distinct instance in any genus; hence on both sides of the sutures, in the sheath of this species, the lines of growth are upturned. In some much disintegrated specimens, both of [C. stellatus] (var. depressus) and of [C. antennatus], the radii have been corroded away, and the diametric growth is effected exclusively by the growth of the alæ, which are moreover much exposed, and rendered conspicuous. The sheath descends a moderate distance down the shell. When a shell is boiled in potash, the sutures (excepting when abnormally calcified together, as very often happens with some species) always fall apart, showing that the union is simply by animalised matter.
Basis.—The basis is always membranous; but we have seen, in [C. intertextus], that the walls form a flat ledge all round the base, and that in old specimens of [C. Hembeli], they grow so far inwards and become so completely confluent, that they might most easily be mistaken for a true calcareous basis. I may add, that in one elongated specimen of [C. stellatus] from La Plata, the walls had likewise grown rectangularly inwards, forming a flat base, and had then turned upwards in the middle, forming a medial crest, with the edges not quite calcified together. The true basal membrane is very obscurely divided into concentric slips. I observed in several species, attached to the lower surface, an excessively fine network, quadrangular or hexagonal, of yellow vessels, which seemed insensibly to pass into the sheets, discs, and globules of cement, by which the membranous basis adheres to the supporting surface. I saw, in [C. antennatus], numerous irregular, bifurcating, and inosculating cement-ducts, of unequal diameter, often crossing each other, and sending off branches ending in points: the older ducts, instead of being solidly filled up with cement, were only divided by septa. I did not succeed, in any species, in discovering the cement-glands.
Mouth.—The labrum is slightly bullate, with the middle portion depressed, but not forming a notch; in some species it is hairy, and in some pectinated with short spines. The palpi are of moderate size. The mandibles have from three to five main teeth, the number sometimes varying even in the same species: the lower teeth are either plainly double laterally, or very obscurely double, or to all appearance quite single: a rather large lower portion of the mandible is finely pectinated. The maxillæ are always notched under the two or three large upper spines: the notch bears some fine spines: beneath the notch there are some large spines, and at the inferior angle some smaller ones.
Cirri.—The first and second pairs are always very short compared with the four posterior pairs. The rami of the first pair are slightly unequal. The third pair, in length and arrangement of the spines, very closely resembles the three posterior pairs; in [C. intertextus], however, the few basal segments, chiefly on the anterior ramus, are thickly clothed with bristles, like the segments of the second cirrus. In [C. antennatus] (Pl. [29], fig. [2]), the anterior ramus of this same third pair is usually (one single specimen being excepted) much elongated, having at least twice as many segments as the posterior ramus, but the number is variable; and these segments, either all, or only the upper ones, instead of having their spines regularly arranged in pairs, in a double row, are surmounted each by a circle of spines: I suspect that these elongated rami of the third cirrus act as antennæ. It can hardly be an accidental coincidence, that certain genera, as Lysmata and Pandalus, amongst the Macrourous Crustaceans, have the same leg (homologically the second thoracic limb) elongated and antenniformed. Certain varieties of [C. stellatus] and [cirratus], also, have the anterior ramus of this same third cirrus considerably elongated. We are thus reminded of the remarkable variability in the numbers of the segments, and in the arrangement of their spines, in the cirri of [Tetraclita porosa]; in that species, however, it was chiefly the terminal segments of the posterior ramus of the third cirrus which were so highly variable. The three posterior pairs of cirri in [Chthamalus] support from three to five pairs of main spines on each segment, the number often varying in the same species, with some minute intermediate bristles. The dorsal surfaces of these segments, in some of the species, are serrated in an upward direction.
Body, &c.—The body does not present any particular character: in [C. scabrosus] there is a slight ridge running from the base of the first cirrus towards the adductor scutorum muscle: this ridge is clothed with a few hairs; there are also some hairs at the carinal end of the sack. In [C. dentatus], also, there are hairs on the outer tunic of the prosoma. In two species which I opened, there were no cæca to the stomach. The ova vary in length from 13 to 14/2000th of an inch in length; they are packed in two lamellæ lying on each side of the animal’s body. The larva just escaped out of the egg, in [C. stellatus], [scabrosus] and [dentatus], had a large probosciformed mouth.
Branchiæ.—These present a very singular amount of difference within the limits of the same genus. In [C. stellatus] and [antennatus] we have a simple fillet, tapering a little, barely plicated, and about half, or more than half, as long as the sack; in [C. scabrosus] the branchiæ are entirely aborted, or are perhaps represented by the slight hairy ridge at the carinal end of the sack: in [C. dentatus], on the other hand, each branchia consists of two large folds, barely plicated, almost covering the whole side of the sack, so that here the branchiæ are developed to an unusual degree, more than in [Balanus], and as in [Coronula] and its allies: in the same manner as in these latter genera, the outer fold is considerably larger than the inner fold.