NOBIA GRANDIS. _G. B. Sowerby, junr._ (sine descript.) Conchological
Manual, fig. 29, 1839.[118]
CREUSIA GRANDIS. _Chenu._ Illust. Conch., Tab. 1, fig. 2 _a_, sed
non, fig. 2.
[118] It is quite possible that this may be the _Balanus
duploconus_ of Lamarck, but with such a character as the following,
who can recognise a species? "_B. testæ parte supremâ univalvi,
indivisâ, convexâ: inferiore turbinatâ, non clausâ: aperturâ
ellipticâ._ L'exemplaire est sans opercule et incomplet."
_Shell moderately convex, nearly smooth: scutum and tergum calcified
together without any suture: scutum furnished with a small occludent
ledge, with the adductor ridge descending below the basal margin:
tergum square without a spur._
_Hab._--Singapore and East Indian Archipelago; Mus. Brit., Cuming,
Stutchbury; imbedded in two kinds of coral.
_Appearance and Structure of Shell and Basis._--The shell is conical,
though to a variable degree, and sometimes is much depressed. The
surface is smooth, with only traces of narrow approximate ridges.
The colour is white, often with a tinge of dark purple. The orifice
is oval, and moderately large. The shell and a small portion of
the basis usually stand exserted above the coral. The walls are
of variable thickness; when thick, the pores, by which they are
permeated, are but little apparent; sometimes there is more than a
single row of pores. The points of the septa on the basal edge of the
shell are small. The internal surface of the shell is smooth. The
sheath is closely attached to the walls, and descends nearly to the
basis; on each side its lines of growth are slightly upturned. The
basis is deeply cup-formed or cylindrical, and in section oval like
the shell; it penetrates the coral to a very remarkable depth,--in
one instance to three inches. The shelly layer forming it, is thin,
finely furrowed, and not permeated by pores. This is the largest
species in the genus; one specimen was three quarters of an inch in
its longer diameter, and above three inches in length.
The _Scuta_ and _Terga_ are calcified together, without any trace
of a suture; the line of junction can be inferred only from the
analogy of _P. conjugatum_, in which species the valves have a more
normal character, and are separated by a slight furrow. It may be
seen in the figure (Pl. 13, fig. 1 _d_) of the right and left hand
opercular valves, viewed from vertically above in their proper
relative positions, how abnormal their appearance is, which is partly
caused by the spinose occludent ledges, presently to be described,
but chiefly from the carinal margins of the two terga not being
straight and parallel, as in all other cirripedes, and therefore not
meeting each other, as is usual. In other genera, only the upper
part of the carinal margin of the two terga can be opened for the
exsertion of the cirri, the lower portion being united by membrane;
but here, I have little doubt, from the position of attachment of
the adductor muscle (fig. 1 _c_), so close to and almost on the
terga, that the whole length of the carinal margin of the two terga
is free or disunited for the protrusion of the cirri. This opening
between the two terga evidently cannot be closed, but is probably
filled up, and the animal thus protected, by the dorsal surfaces of
the curled-up cirri; such, I believe, being likewise the case with
some pedunculated cirripedes, as with Conchoderma. The _scutum_ has
a large adductor plate, which extends some little way (namely, about
one quarter of the height of the valve), below the ordinary basal
margin. This latter margin is slightly sinuous, and a little hollowed
out towards the tergal corner of the valve. I believe that the ridge,
which runs down to the basi-scutal corner of the tergum, though
appearing to be part of the scutum, really belongs to the tergum.
The adductor scutorum plate is not, as in the last two species,
produced into a point at the rostral angle; at the tergal end it
blends into the under surface of the tergum. The occludent margin
is coarsely toothed. Rather on the under side of this margin and in
the upper part, there is a narrow occludent ledge, which extends up
beyond the apex of the valve, and thence runs a little way along the
carinal margin of the tergum. This ledge is thickly clothed with
strong, yellowish-brown spines, and hence appears like a brush. It
is remarkable that the cavity for the adductor scutorum muscle is
situated almost on the tergum.
The _tergum_ is of large size, and nearly square; it is, in
appearance, separated from the scutum by a ridge running up to the
apex. The basal margin forms a right angle with the carinal margin,
along which latter margin the lines of growth are upturned, and blend
into the occludent ledge, which is common to the two valves. There
cannot be said to exist any spur, the whole basal margin being almost
straight; nevertheless, on close examination, the ridge which in
appearance separate the scutum and tergum, may, I think, be safely
considered as one side of the spur (which, it should be remembered,
has in all ordinary cases a longitudinal furrow or depression), and
the other side of the spur is, apparently, very feebly indicated by a
slight flexure in the middle of the basal margin. Hence, if the spur
had been developed, it would probably have been half as wide as the
valve. There are no crests for the tergal depressor muscles.
_Affinities._--The present species, with the last two, form an
interesting series. _Pyrgoma grande_ and _conjugatum_, however,
are more closely allied to each other than to _P. cancellatum_. In
the scutum, the whole valve is least elongated, with the adductor
plate least developed, in _P. grande_, and most elongated, with
the adductor plate most developed, in _P. cancellatum_. In the
outline of the tergum the range of shape is quite remarkable; in _P.
conjugatum_, which stands between the other two species, the spur is
rather long, whereas in _P. grande_ there is no spur at all--a very
unusual circumstance--and in _P. cancellatum_, at the other end of
this short series, the spur attains a length wholly unparalleled in
any other cirripede.