[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.