LEPAS DIADEMA. _Linn._ Systema Naturæ, 1767.
---- ---- _Chemnitz._ Conch., vol. 8, Tab. 99, figs. 843, 844.
BALANUS DIADEMA. _Bruguière._ Encyclop. Method., n. 164, fig. 13, 14
(1789).
CORONULA ---- _De Blainville._ Dict. des Sc. Nat. (1824), Tab. 117,
fig. 4.
---- ---- _Leach._ Encyclop. Brit. Suppl., vol. iii, 1824.
---- ---- _Chenu._ Illust. Conch., Plate, fig. 3.
---- ---- _Burmeister._ Beiträge zur Naturgeschichte der
Rankenfüsser, 1834, Tab. 2, fig. 1-14, 18.
_Shell crown-shaped, with longitudinal convex ribs, having their edges
crenated; orifice hexagonal: radii moderately thick, very broad: terga
absent or rudimentary._
_Hab._--Attached to whales, in the Arctic Seas; United States and
Great Britain; Gulf-Stream, Atlantic Ocean; New Zealand(?).
_General Appearance._--As previously remarked, owing to the fulness
of the generic description, minute details on structure, excepting
those characteristic of the present species, need not here be given.
The crown-like shape of the shell is well expressed by its name of
_Diadema_, but the crown tends to pass into a cylinder. The radii are
extremely broad. The orifice is large, and neatly hexagonal: when
the operculum is removed the whole inside of the cup-formed shell
can be seen at once, for the flat membranous basis is much smaller
than the orifice. The under side of the shell is deeply concave.
The outside of each compartment is formed by broad, rounded, and
somewhat prominent, rarely divided, ribs (_i. e._ the transverse
ends of the folded walls); these ribs are closely united together
by finely serrated lines of junction (Pl. 16, fig. 1, _f_). Their
surfaces outside are delicately striated longitudinally, and plainly
crossed (more plainly than in the foregoing and the next species) by
irregular, transverse ridges, especially in the lower part of the
shell. The largest specimen which I have seen was two and a half
inches in diameter and two in height.
_Scuta._--These are placed close together at the rostral end of the
orifice, and are imbedded in the brownish, tough, longitudinally
plicated, horny substance, which extends far beyond both ends of the
valves. In outline (fig. 3 _b_) they are mitre-shaped, or rounded
and sub-triangular, a little curved, and more or less elongated,
being most so in young specimens; they are, however, less elongated
and rather more massive than in _C. balænaris_. _Terga_,--these seem
entirely absent in most specimens; but in one (fig. 3 _a_) I found
a rudiment, namely, a short thin plate of shell, barely visible to
the naked eye, extending parallel and near to the tergal margin of
the scutum. The lips of the aperture of the sack are prominent, and
highly so towards the carinal end.
_Structure of Shell and Radii._--Owing to the shell not spreading
much at the base, new folds in the walls are much seldomer formed,
and therefore the external longitudinal ribs (_i. e._ the terminal
transversely elongated loops), are much seldomer divided, than in _C.
balænaris_ or _reginæ_; even rather large specimens sometimes having
only the original eighteen folds.[125] When new folds are formed,
only one is formed on one, viz., the rostral, side of each suture,
instead of on both sides, as in _C. balænaris_. The inner ends of the
folded walls, surrounding the basal membrane, are narrow, instead
of being almost square, as in _C. balænaris_. The lower edge of the
sheath, which projects freely, descends almost to the level of the
basal edges of the walls. The outer ends of the folded walls, forming
the transverse loops, are internally filled up solidly by calcareous
matter, instead of by septa forming tubes, as in _C. balænaris_.
The radii (Pl. 16, fig. 2) are a little thicker in the lower and
outer than in the upper and inner part of each compartment; in the
middle, they do not reach the sheath by about half the thickness of
the compartment, and consequently they are separated from the plates
(_c′′_ in fig. 1 and 7) on which the alæ rest, by large chambers (_v_
in fig. 7), which extend up to nearly the apices of the compartments:
the extent, however, to which the upper ends of these chambers have
been solidly filled up, varies a little. The sinuous plates forming
the main portion of the compound radii are rather thinner and closer
together than in _C. balænaris_. The alæ are thick, being thickest in
the middle part, and there equal the radii in thickness; their lower
margins are very short compared with their upper margins, hence they
are almost wedge-formed.
[125] These should be counted on the under side of the shell,
for if counted from the outside, the number would be thought to
be twenty-four, as on the side of each compartment bordering the
radius, a half-fold has the appearance of being a whole fold, so
that in appearance six folds are added to the real number of the
folds. This caution is necessary whatever the number of real folds
may be, that is, whether or not the original eighteen folds have
been increased by subsequent plications. These remarks, also, are
applicable to the other species.
_Mouth._--The teeth and fine hairs on the labrum are sometimes
obscure, and sometimes plain: close outside the bottom of the medial
notch, there is a small hard prominence. The palpi are broad; on
their basal exterior margins there is a short row of spines, which
do not equal in length the width of the palpi, and therefore are not
so long as in _C. balænaris_. The mandibles have five main teeth, of
which the second and third show only an obscure rudiment of being
double; between these two teeth, and between the third and fourth
tooth, there is a small intermediate tooth: the inferior angle is
narrow, rounded, and spinose. The extremity of the apodeme of the
maxillæ is expanded.
_Cirri._--I have only to remark, that the pedicel of the first cirrus
is extremely broad, and that the rami are set on in an unusually
crooked manner: the basal segment of the shorter and broader ramus of
this cirrus has its dorsal surface produced into a plate fringed with
very fine hairs.
_Geographical Distribution._--I have received only four specimens
with certain localities attached to them: namely, the Arctic Seas of
Scandinavia; the coast of the United States, and of Britain; and the
Gulf-Stream. There is also a specimen in the British Museum, sent by
Mr. Stephenson, mingled with shells of Mollusca from New Zealand;
but a Coronula, procured from a floating whale in the early part
of the outward voyage, might so easily be sent home with specimens
subsequently collected in another county, that I do not as yet fully
admit that this species is an inhabitant of the Southern Pacific
Ocean: I am less willing to admit this, from suspecting that _C.
reginæ_ in the Pacific, replaces the _C. diadema_ of our Northern
Seas.