[133] Παχυς [Greek: Pachus] thick, and ελασμα [Greek: elasma], a valve.
CHTHAMALUS. Philippi. Enumeratio Mollusc. Siciliæ.
Compartments, when the shell is very young, eight; when mature, either six, or in appearance only four owing to the close union of the lateral compartments: basis calcareous.
Distribution, Mediterranean, and New South Wales; deep water.
The two species here included form a very natural genus, though, as far as the shell alone is concerned, at first sight there is an unusual amount of difference between them. This genus offers an instance of a case, far from uncommon in nature, though so unfortunate for the systematist, in which the most obvious and useful characters of a group are completely masked. When I first examined Pachylasma giganteum,[134] I did not doubt that it was a Balanus; and when I first looked at P. aurantiacum, I thought, from there being in appearance only four compartments, that it was an Elminius; in neither case, from the absence of alæ to the rostrum, did I even suspect that the species belonged to the sub-family of the Chthamalinæ. But when I examined the included animal’s body, I found, in both species, the labrum bullate, not notched, with the palpi small, and the mandibles with their lower teeth not laterally double. Again I found in the third pair of cirri only the basal segments thickly clothed with spines; and lastly, there were caudal appendages. Now these characters are pre-eminently those of the Chthamalinæ; in fact, they are those met with in the typical genus Octomeris, with the exception of the presence of caudal appendages, and these occur in Catophragmus,—a genus standing next to Octomeris, and in no other genus of sessile cirripedes. Moreover, if we look to the shell of Pachylasma, the absence of pores in the parietes, or at least of symmetrical longitudinal ribs on their inner surfaces, and the peculiar character of the narrow radii, hardly differing in structure from the parietes, are characters which are rare in the Balaninæ, but universal in the Chthamalinæ. Hence, taking the whole organisation of the two species of Pachylasma, it is certain that they must be ranked amongst the Chthamalinæ, though the leading character of the group, namely, the rostrum being furnished, like the carina, with alæ, here fails. Owing to this conviction, I examined very young individuals of P. giganteum, and in specimens only the 1/100th of an inch in height, I was interested by finding eight separate compartments, with the rostrum having distinct alæ; hence, at this early age, as far as the shell is concerned, this species may be said to be an Octomeris; and we have seen that this likewise holds good with the included animal’s body; as the young shell increases in size the minute rostrum and rostro-lateral compartments blend together (Pl. 19, fig. 5 b), without even traces of sutures being left.
[134] Dr. Philippi called this species a Chthamalus; giving this generic name from an examination only of the separated valves in a fossil condition.
With respect to Pachylasma aurantiacum, I have no doubt that at an early age it would possess a perfectly distinct rostrum with alæ; for in the one specimen which I have seen, the compound rostrum is divided by sutures, faintly visible, both externally and internally (Pl. 20, fig. 1 a, 1 b), into three compartments (B, A, B), of which the middle one, or true rostrum, still shows, in the manner in which it underlaps the little rostro-lateral compartments, vestiges of alæ. I may remark, that we have here the same structure as in Chelonobia, formerly described, with the following differences, that here the sutures pass through the outer lamina of the parietes, so that, as seen externally, the separation of the three compartments is much more perfect than in Chelonobia; on the other hand, internally, the separation is less distinct, as the two rudimentary rostro-lateral compartments do not form part of the sheath. Pachylasma aurantiacum is further remarkable, from the two lateral compartments (C, D) on each side, tending to blend together, being only separated by sutures not more distinct, externally, than those separating the compound rostrum, but more distinct internally, for they run up the sheath. From these facts it follows, that this species, viewed outside, without particular care, would be said to consist, like an Elminius or Chamæsipho, of only four compartments; if the sheath alone were looked at, there would be said to be six compartments; but when the walls, especially their basal edges, are carefully examined, either internally or externally, the eight compartments can be plainly distinguished.
With respect to the affinities of this genus, we have seen that both in the included animal’s body, and, at an early age, in the shell, it is extremely close to Octomeris: in the presence of caudal appendages, and in the basis being calcareous, we have a clear affinity with Catophragmus; in the two lateral compartments of P. aurantiacum, tending to become blended together, we have some relationship exhibited to Chamæsipho, as is likewise shown in the structure of the second and third pairs of cirri. If the genera of the Chthamalinæ were ranged in a circle, Pachylasma would be the point of contact with the Balaninæ. I must repeat, that it is extremely unfortunate that when the shell alone of P. giganteum is examined, it is hardly possible to separate this genus from Balanus.
CHTHAMALUS GIGANTEUS. Philippi (!). Enum. Mollusc. Siciliæ, 1836.
Shell and operculum dirty white: carino-lateral and lateral compartments furnished with similar alæ.
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.
Cirri: first pair short, with the rami equal in length. Second pair, with the anterior ramus having broader segments than those of the posterior ramus, and with all the segments, except the few uppermost, thickly covered with spines; the posterior ramus has rather less than half the segments thickly covered. Third pair, very slightly shorter than the sixth pair; anterior ramus with the lower segments, less than half of the whole in number, thickly covered with spines; posterior ramus with only the lowest segments, about one fifth of the entire number, thickly covered; the other segments of these two rami, and the upper segments of the posterior ramus of the second pair, closely resemble in the regular arrangement of their spines in pairs, the three posterior pairs of cirri. The pedicel of the third pair supports numerous, irregularly scattered bristles. The segments of the sixth pair bear four or five pairs of main spines, with a few intermediate spines.
Caudal Appendages.—Multiarticulate, narrow, tapering, situated on each side of the anus: each segment has two little tufts of spines on each side of its upper edge. These appendages are about one third longer than the pedicel of the sixth pair of cirri: in a specimen, in which the rami of the sixth pair had twenty-three segments, the caudal appendages had nineteen segments.
Penis short, hairy, finely-ringed, with no projecting point at its dorsal basis. Branchiæ moderately large, nearly circular, not much plicated.
Fossil Specimens.—With respect to the Sicilian specimens sent me by Dr. Philippi and Sir C. Lyell, I can see no difference whatever from the recent specimens, excepting in their greater size and thickness; it must, however, be borne in mind that I have seen only half-a-dozen recent shells. The one fossil scutum which I have seen is rather broader than is usual with the recent, but I have seen one nearly as broad. This species seems to have been extremely common when the beds at Messina were deposited, and probably it attained a larger size than it does at present. The compartments are always found separated, which is easily understood, by the facility with which, as above stated, recent specimens fall to pieces.
Shell tinged with orange; viewed externally seems formed of only four compartments, owing to the carino-lateral and lateral compartments on each side being separated only by an obscure fissure.
Hab.—New South Wales; apparently from deep water, attached to sandstone.[135]
[135] I am indebted to Mr. Bowerbank for this unique and interesting species, which I have deposited in the British Museum.
General Appearance.—Shell conical, smooth, with a tinge of orange colour; orifice large, deeply notched, sub-triangular. Viewed externally, the compartments in appearance are only four; but on close examination, the lateral compartments are seen to be divided by a very fine fissure into two nearly equal compartments. The rostrum is broad and flat, and when carefully examined, it also is seen to be divided by two fine fissures into three compartments; of these the middle one, or true rostrum, is a very little broader than the rostro-lateral compartment on each side; hence, on careful examination, the shell is found to consist of eight compartments. The carina is much compressed. The radii (in my one specimen) are not developed. The carina, and the two lateral compartments alone, have alæ; for the carino-lateral compartments are too closely joined to the lateral compartments, and the true rostrum is too closely joined to the rostro-lateral compartments, to have their alæ developed. The four alæ which are developed, are very broad, widely exposed, and marked externally by lines of growth. The basal diameter of my one, apparently old specimen, is one inch.
Structure of Shell, &c.—The inner surface (fig. 1 b) of the rather thick parietes is smooth, except close to the base, where it is roughened by a few irregular points. The sheath descends low down, and has its lower edge slightly free. The sutures are much plainer on the internal than on the external surface, and can be here plainly seen to be eight in number. That portion of the carino-lateral compartment, which helps to form the sheath, is narrow, though the whole compartment is of very nearly equal width with the lateral compartment. It is a singular fact, that no portion of the rostro-lateral compartment helps to form the sheath; for the alæ of the lateral compartments, overlap the whole upper part of the rostro-lateral compartments, and abut against the true rostrum. Hence, when the sheath alone is examined, the number of the compartments appears only six. In a section the true rostrum can be seen to underlie the rostro-lateral compartments, and thus exhibits vestiges of alæ. The fact of the rudimentary rostro-lateral compartments not forming a part of the sheath offers a marked difference from Chelonobia, which otherwise has this part of the shell very similarly constructed. The diametric growth of the shell, which seems to be considerable, is effected by the four large alæ of the carina and of the lateral compartment on each side. The sutural edges of the alæ are added to in a regularly inclined line down to the basis. The basis is calcareous, and not very thin.
Both opercular valves (fig. 1 c, 1 d) closely resemble those of the last species.
The Scuta are remarkably narrow and elongated: the external surface is slightly furrowed longitudinally, the prominent lines of growth are much wrinkled. The articular ridge is blunt, and not very prominent. The upper part of the valve is reflexed, and a considerable portion must have projected freely. The Terga are nearly twice as broad as the scuta: the carinal half of the valve is very smooth, and is coloured beautiful reddish-orange. The spur is tolerably distinct. The articular ridge is not very prominent.
The Mouth resembles that of the last species. The mandibles have three sharp teeth, with the inferior part narrow and pectinated with long spines: the edge, also, between the upper main teeth, is pectinated with short spines, which latter often have their summits crenated. The maxillæ are notched; the inferior corner is produced into a small step-formed projection. The outer maxillæ are more pointed than is usual.
The Cirri most closely resemble in every detail those of P. giganteum; I can point out only one slight difference, namely, that the lower segments, in the posterior ramus of the third cirrus, which are thickly clothed with spines, are more numerous in proportion to the upper segments with the spines arranged in regular pairs (being as 4 to 14), than in P. giganteum, in which only one fifth (or 4 to 20) are so clothed.
Caudal Appendages.—These are very small, not being more than one fifth of the length of the pedicel of the sixth cirrus: their segments are indistinct, and they support a very few coarse spines. The rami of the sixth cirrus, in the one specimen, had twenty-five segments, whilst each caudal appendage had only five. Hence the caudal appendages are far less developed than in P. giganteum.