[119] From external aspect I suspect this species to be the
_Creusia madreporarum_, Leach (?), as given in Chenu, 'Illust.
Conch.,' Tab. 1, fig. 6. But I feel sure that Leach has nowhere
published this name; and it may be observed that Chenu gives it
with a mark of doubt. The shell in its imbedded state is only
figured; the opercular valves are not given; and no descriptive
details are added. Under these circumstances I have not adopted
this name; I have, perhaps, been in some degree influenced by the
fact that this species, judging from the many specimens examined by
me, is never imbedded in madrepores, but exclusively in millepores,
so that Chenu or Leach's specific name of _Madreporarum_ is
singularly inappropriate.
_Shell with the orifice narrowly ovate: sheath dark purple: Scutum much
elongated: tergum triangular, convex, without a spur._
_Hab._--Philippine Archipelago (Mindoro Island), Mus. Cuming. Mus.
Brit., Stutchbury, &c. Imbedded in _Millepora complanata_, sometimes
associated with _Balanus Ajax_.
_Appearance and Structure of Shell._--Shell oval, flat, coloured pale
dull purple, or white, with slight and narrow ridges, radiating, from
the orifice, which is not quite central, but is placed rather nearer
to the carinal than to the rostral end of the shell. The orifice is
small and narrow; the carinal end being rounded, and the rostral end
narrower and sharper,--this being the exact reverse of the usual
shape of the orifice in the Balanidæ. The walls are thick, and are
formed of large square tubes. The internal surface of the shell is
smooth. The sheath (fig. 2 _b_) is much more elongated than the
shell, for at the rostral end it extends to the basal margin, and at
the carinal end nearly to it, whereas on the two sides it is some way
distant from the base. The orifice is considerably out of the centre
of the sheath, being placed nearest to the carinal end. The sheath
has its lower edge slightly prominent or free; the lines of growth
are neither turned up nor down on the two sides, as in the last three
species. When nearly full grown the sheath is coloured dark purple,
but when young it is white, hence the upper part is white, surrounded
by an oblong purple ring, and this is surrounded by the white shell.
The basal cup is deep, and internally nearly smooth. The largest
specimens were .3 of an inch in their longer diameter. Great masses
of the Millepora are absolutely studded with this Pyrgoma, with
usually more specimens on one side of the plate or branch than on the
other. They stand in approximately parallel positions, the broad or
carinal ends of the orifices pointing upwards.
_Scuta._--The scuta and terga are closely united, and _are_ often
(perhaps always) in some slight, though variable degree, calcified
together; and hence they often break, rather than separate, at the
line of articulation. The external fissure or line of junction
between them (fig. 2 _c_) is oblique to the longer axis of the
scutum; in the uppermost part of the valve it is sometimes almost
obliterated. The two valves together are nearly as long as the
sheath, and consequently much longer than the orifice of the shell.
The scutum is much elongated, being fully four times as broad as
high. The valve narrows towards the rostral end, but in a variable
degree: the basal margin is hollowed out a little (but to a variable
amount), close to the basi-tergal corner. Along the occludent margin
a slip of the valve, widening downwards, is a little bent inwards,
and this inflected portion is separated from the rest of the surface
by a slightly angular ridge, running from the apex to near the
rostral angle. Internally, at the basi-tergal corner, a slight ridge,
parallel and close to the basal margin (and which can be seen only
when the basal edge of the valve is held upwards), represents the
adductor ridge, which we have seen so largely developed in the last
three species, and shall again see in _P. crenatum_. The articular
ridge (fig. 2 _d_) is extremely prominent, consisting of a more or
less rectangular shoulder.
_Terga_: these are rather small compared to the scuta: they are
triangular and much arched: there is no trace of a spur. Internally
(fig. 2 _f_), the articular ridge is central: there are some vestiges
of crests for the depressor muscle.
_Affinities._--This well-marked species, in the tendency of the
opercular valves to be soldered together, and in the remarkable
absence of a trace of a spur to the tergum, is allied to _P. grande_,
but it is more closely allied to the three following species.