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.