Hab.—Mediterranean; Sicily; deep water; often attached to the Millepora aspera, and sometimes associated with [B. tulipiformis]. Fossil in the tertiary beds, near Messina. Mus. Brit., Cuming, Stutchbury, Lyell, Philippi.
I owe to the kindness of Dr. Philippi, authentic specimens of his Chthamalus giganteus, from the tertiary beds of Messina: had this distinguished naturalist seen recent specimens, or a fossil one with all the valves united, he would no doubt have perceived that this species cannot be classed with [Chthamalus]. I am also indebted to Sir Charles Lyell for some magnificent specimens, which he himself collected near Messina.
General Appearance.—Shell conical, rugged, irregular, with the lines of growth plain; colour dead dirty white. Orifice large, diamond-shaped, narrow towards the carinal end; notched. Radii narrow, barely distinct from the parietes; in the same individual sometimes absent, and sometimes forming a mere ribbon, confined to the lower edge of a compartment. The diametric growth is effected by the alæ, which seen externally are broad, and strongly marked by lines of growth. Basal diameter of largest recent specimen 1.15; height of highest compartment (a carina) 1.4; and width at base of widest rostrum .85 of an inch. Amongst the fossil specimens, height of highest compartment (a carina) 1.8, width of the same one inch; width of widest rostrum 1.1 of a inch. One of these carinæ, a little below the middle point, was actually .3 of an inch in thickness, from which circumstance I have given the generic name of [Pachylasma].
Scuta, triangular, but the width varies a little: growth-ridges prominent, sinuous, with a few slight furrows radiating from the apex of the valve. Internally, the articular ridge is not very prominent; nor is there a deep depression for the adductor muscle, and none for the lateral depressor muscles. The upper part of the valve projects freely.
Terga, broad, broader than the scuta: the growth-ridges are prominent, and angularly upturned close along the scutal margin; the carinal half of the valve is smooth, with the faintest traces of longitudinal striæ. A portion of the valve, nearly half of its entire width on the scutal side, is slightly depressed below the general level, and depends slightly beneath the basal margin on the carinal side: this evidently forms the spur. The carinal margin is nearly straight, with the lines of growth upturned along it. Internally, the articular ridge in the upper part is extremely prominent: the crests for the depressor muscles are very prominent, and depend beneath the basal margin like a comb: they extend over nearly half the basal margin, and the muscle, in a corresponding manner, is unusually spread out.
Structure of Shell and Radii: the compartments are attached to each other less strongly than in any other cirripede which I have examined, so that when dried specimens are soaked in spirits of wine they generally fall to pieces with a touch. In full and half-grown specimens the carino-lateral compartments are nearly as broad as the lateral compartments; in very young specimens, about 1/20th of an inch in basal diameter, they are proportionally much narrower. The walls are strong even in young specimens; in old ones they attain a thickness I have scarcely seen equalled except in [Chelonobia]. Their internal surfaces are smooth, as is the basal internal margin in young specimens, but in old specimens it is roughened with short, blunt ridges and little points. The Radii are often absent; when present they are very narrow, and consist merely of a ribbon-like portion, formed by obliquely upturned layers of growth, more prominent than on the parietes. The Alæ are very largely developed; they are added to, during the diametric growth of the shell, in a regular sweep all the way down to the basal margin, and consequently they do not form a rectangular shoulder as is usual; externally they are plainly marked by lines of growth: they are added to a little above the line of attachment of the opercular membrane; their summits are very oblique. The sheath has its basal edge slightly hollowed out.
Basis, solid, calcareous, very irregular, and of variable thickness.
I have, under the Genus, alluded to the structure of the rostrum: in one shell, the basal diameter of which barely exceeded 1/20th of an inch; the compound rostrum (being 5/200ths of an inch in height), had its basal margin (being 9/200ths of an inch in width) rendered deeply sinuous (see Pl. [19], fig. [5 b]) by two indentations, corresponding with and caused by two notches at the top of the valve. These two notches extended down barely 1/100th of an inch (strictly 4/1000ths) from the summit and then disappeared; so that when the shell was under 1/100th of an inch in height, (only one distinct zone of growth having been formed), the now compound rostrum consisted of three separate compartments; and there were eight compartments altogether. Of the above three little compartments, the middle one, or true rostrum, had large alæ, which could be most distinctly seen, extending on both sides, under the little rudimentary rostro-lateral compartments. These latter overlapped the compartments on both sides of them, as in all the [Chthamalinæ].
Mouth: labrum bullate, with no central notch; nor is the inner fold of the labrum, forming the supra-œsophageal cavity, thickened, as in [Balanus]: minute muscles run from this inner fold straight back to the cavity formed by the outer bullate fold: the crest of the labrum is hairy, with a row of the minutest bead-like points or teeth. Palpi, small, broad, placed almost parallel to the sides of the mouth, with their apices not nearly touching each other. Mandibles, with three large nearly equal-sized single teeth; the whole inferior angle strongly pectinated. Maxillæ, small, with a broad, square notch beneath the two or three great upper spines.