the scuta, there is a strong affinity between the present and following
species; for we shall immediately see that in _P. eburnea_ there is
evidence of the scuta being composed of two segments fused together; and
the larger segment is furnished with an internal oblique, strong, basal
rim. To this same species there is an evident affinity in the form of
the mandibles and of the caudal appendages, and in the anterior rami of
the cirri being longer than the posterior rami. Notwithstanding these
points of affinity, I consider that _P. fissa_ is more closely related
in its whole organisation, as more particularly shown in the arrangement
of the spines on the cirri and in the presence of terga, to _P. crassa_
than to _P. eburnea_. Although in Dichelaspis, the scuta are invariably
composed of two almost separate segments, yet _P. fissa_ shows no
special affinity to this genus.
5. PÆCILASMA EBURNEA. Pl. II, Fig. 5.
TRILASMIS EBURNEA. _Hinds._ Voyage of Sulphur, 1844, vol. i,
Mollusca, Pl. xxi, fig. 5.
_P. valvis 3; scutis acuminatis, ovatis; ad pedunculum pæne transversè
spectantibus; dentibus internis umbonalibus fortibus: tergis nullis:
carinæ termino basali in discum amplum oblongum infossum producto._
Valves 3; scuta pointed, oval, placed almost transversely to the
peduncle; internal umbonal teeth strong: terga absent: carina with the
basal end produced into a large, oblong, imbedded disc.
Spines on the upper segments of the posterior cirri, arranged in three
or four approximate longitudinal rows, making small brushes.
_Habitat._--New Guinea, attached to the spines of a dead
Echinus. Brit. Mus., and Cuming.
_General Appearance._--Capitulum flat, pear-shaped, placed almost
transversely to the peduncle. Valves white, smooth, moderately thick.
_Scuta:_ the basal margin, as seen externally, is narrow, and can hardly
be separated from the carinal margin; but an internal basal rim, (fig.
5, _b_) (along which the imbedded disc of the carina runs,) shows where,
in the other species, the basal and carinal margins are separated. This
basal internal rim is not parallel to the external basal margin, but
runs upwards to the occludent margin, leaving beneath it a large
triangular space, to which the membrane of the peduncle is attached; and
this makes it appear as if the rostral umbones of these valves had grown
downwards; but, judging from the allied species, _P. fissa_, I have no
doubt that the primordial valves really lie on the umbones, and that the
growth has been in the usual direction, that is, exclusively upwards.
The occludent margin is curved, and blends by a regular sweep into the
carinal margin, so that there is no acute upper angle. A distinct line
can be seen, as if two calcareous valves had been united, running from
the umbo to the upper end of the valve, thus in appearance separating a
slip of the occludent margin; internally this appearance is more
conspicuous; this structure is important in relation to that of _P.
fissa_. The pointed umbones are divergent, and internally under each,
there is a large tooth. The two valves are equally convex.
_Terga_, entirely absent.
The _Carina_ (Tab. II, fig. 5, _a_, _c_), including the disc, is three
fourths as long as the scuta; it is placed almost transversely to the
longitudinal axis of the peduncle; it is narrow and internally convex;
the imbedded disc is very large, forming a continuous curve with the
upper part of the carina; this disc runs along the internal basal rim of
the scuta, and hence almost separates, internally, the peduncle from the
capitulum; it equals one fourth of the total length of the valve, and is
thrice as wide as the upper part; it is oval, externally marked by a
central line, and with a slight notch at the end, giving a divided
appearance to the whole, and indicating how easily a fork might be
formed from it. The carina is thick, measured from the inner convex to
the exterior surface, which is carinated; heel prominent.
_Peduncle_, narrow, very short, not nearly so long as the capitulum.
_Mouth._--Labrum considerably bullate, with the lower part much produced
towards the adductor muscle; crest with small bead-like teeth; palpi
small, pointed; mandibles, with the first tooth standing rather distant
from the second; inferior angle spine-like and bifid; maxillæ (Pl. X,
fig. 15), with two considerable spines (only one is shown in the Plate)
beneath the upper large pair; the inferior upraised part bears seven or
eight pair of spines, and its edge is not quite straight; close to the
main notch, lying under the four upper spines, there are two minute
notches, with the interspace bearing a tuft of fine spines and a pair of
larger ones.
_Cirri._--The rami in all are rather unequal in length, the anterior
rami being rather the longest; the anterior rami of the second and third
cirri are not thicker than the posterior rami. The segments in the three
posterior cirri are not protuberant; the upper segments bear three or
four pair of spines, with some minute intermediate ones, and with the
lateral marginal spines unusually large and long, so as to form, with
the ordinary pairs, a third or fourth longitudinal row; hence a small
brush is formed on each segment. The dorsal tuft is large and wide, so
as to contain even fourteen spines, of which some are as long as those
in front. In the lower segments of these same posterior cirri, the
lateral marginal spines are not so much developed (nor is the dorsal
tuft), and hence the segments can hardly be said to be brush-like. The
first cirrus is placed rather distant from the second pair. The second
and third cirri differ from the three posterior pair, only in the
bristles being slightly more numerous, and in the dorsal tufts being
more spread out.
_Caudal Appendages_ about half the length of the lower segments of the
pedicels of the sixth cirrus; truncated and rounded at their ends;
thickly clothed with long excessively fine bristles, so as to resemble
camel-hair pencils.
The _Stomach_, I believe, is destitute of cæca; in it was a small
crustacean.
_General Remarks._--I was at first unwilling to sacrifice Mr. Hind’s
genus, Trilasmis, which is so neatly characterised by its three valves;
moreover, the present species does differ, in some slight respects, from
the other species of Pæcilasma; but under the head of _P. fissa_ I have
shown how that species, _P. crassa_ and _P. eburnea_ are tied together.
The absence of terga, which are rudimentary in _P. crassa_, (and we
shall hereafter see, in _Conchoderma_, how worthless a character their
entire absence is,) and the arrangement of the spines in the upper
segments of the posterior cirri, are the only characters which could be
used for a generic separation.
_Genus_--DICHELASPIS. Plate II.
OCTOLASMIS.[32] _J. E. Gray._ Annals of Philosophy, vol. x, new
series, p. 100, August 1825.
HEPTALASMIS. _Agassiz._ Nomenclator Zoologicus.
_Valvæ 5, quæ ferè pro septem haberi possent, scuto in segmenta planè
duo, ad angulum autem rostralem conjuncta, diviso: carina plerumque
sursum inter terga extensa, deorsum aut disco infosso aut furcâ aut
calyce terminata._
[32] From [Greek: dichêlos], bifid, and [Greek: aspis], a shield,
or scutum. The name Octolasmis was given by Mr. Gray under the
belief that there were eight valves. Leach (as stated in the
‘Annals of Philosophy,’) had proposed, in MS., the name
Heptalasmis, and this is now used in the British Museum by Mr.
Gray, and thus appears in Agassiz’s ‘Nomenclator Zoologicus.’
Although, strictly, there are only five valves, I continued to
use, in my MS., the term Heptalasmis, until I examined the _D.
orthogonia_, where it was so apparent to the naked eye that there
were only five valves, the scuta in this species being less
deeply bifid, that I was compelled to give up a name so
manifestly conveying a wrong impression, and hence adopted the
one here used.
Valves 5, generally appearing like 7, from each scutum being divided
into two distinct segments, united at the rostral angle; carina
generally extending up between the terga terminating downwards in an
imbedded disc, or fork, or cup.
Mandibles, with three or four teeth; maxillæ notched, with the lower
part of edge generally not prominent; anterior ramus of the second
cirrus not thicker than the posterior ramus, not very thickly clothed
with spines; caudal appendages uniarticulate, spinose.
_Distribution._--Eastern and Western warmer oceans in the
Northern hemisphere, attached to crustacea, sea-snakes, &c.
_Description._--The capitulum appears to contain seven valves; but, on
examination, it is found that two of the valves on each side, are merely
segments of the scutum; these are united at the umbo, in three of the
species, by a narrow, non-calcified portion of valve, where the
primordial valve is situated; in _D. orthogonia_, however, the junction
of the two segments is perfectly calcified, and of the same width as the
whole of the basal segment. The capitulum is much compressed, broad at
the base, and extends a little beneath the basal segments of the scuta.
The valves are very thin, often imperfectly calcified, and generally
covered with membrane. They are not placed very close together, and in
all the species a considerable interspace is left between the carina and
the two other valves: in the _D. Grayii_ the valves are so narrow that
they form merely a calcified border round the capitulum. The membrane
between the valves and over them, is very thin, and is thickly studded,
in some of the species, with minute blunt conical points, apparently
representing spines. The valves in the same species present considerable
variations in shape; in their manner or direction of growth, and in the
position of their primordial valves, they agree with Lepas and
Pæcilasma.
_Scuta._--In three of the species the two segments, named the occludent
and basal, appear like separate valves, but these, by dissection, can be
most distinctly seen to be united at the rostral angle. The primordial
valve, formed of the usual hexagonal tissue, is elliptic, elongated, and
placed in the direction of the occludent segment; calcification
commences at its upper point, so as to form the occludent segment, and
afterwards at its lower point, but rectangularly outwards, to form the
basal segment; in the minute space between these two points of the
primordial valve, there is, in four of the species, no calcification; so
that the two segments are united by what may be called a flexible hinge;
in _D. orthogonia_ the two calcareous segments are absolutely
continuous. The occludent segment is longer than the basal segment; it
either runs close along the orifice, or in the upper part bends inwards;
both segments are narrow, except in _D. Warwickii_, in which the basal
segment is moderately broad; the two segments are placed at an angle,
varying from 45° to 90°, to each other. The capitulum generally extends
for a little space beneath the basal segments of the scuta, where it
contracts to form the peduncle.
The _Terga_ present singular differences in shape, and are described
under the head of each species; scarcely any point can be predicated of
them in common, except that they are flat and thin.
The _Carina_ is much bowed, narrow, and internally either slightly
concave or convex and solid; the upper end extends far up between the
terga; the lower end is formed by a rectangularly inflected, imbedded,
triangular or oblong disc, deeply notched at the end, or as in _H.
Lowei_, of a fork, the base, however, of which is wider than the rest of
the carina, so as to present some traces of the disc-like structure of
the other two species; or lastly, as in _D. orthogonia_, it terminates
in a crescent-formed cup.
_Peduncle._--This is narrow, compressed, and about as long, or twice as
long, as the capitulum; in _D. Warwickii_ it is studded with minute
beads of yellowish chitine.
_Size._--Small, with a capitulum scarcely exceeding a quarter of a inch
in length.
_Filamentary Appendages._--None. There are two small ovigerous fræna,
which, in _D. Warwickii_, had the glands collected in seven or eight
little groups on their margins.
_Mouth._--Labrum highly bullate, with small teeth on the crest; palpi
small, not thickly covered with spines. _Mandibles_ narrow, with three
or four teeth. Maxillæ small, with a notch beneath the two or three
great upper spines; lower part bearing only a few pair of spines,
generally not projecting, but in _D. orthogonia_ largely projecting.
Outer maxillæ, with their inner edges continuously covered with
bristles.
_Cirri._--First pair short, situated rather far from the second pair;
second pair with the anterior ramus not thicker than the posterior
ramus, and hardly more thickly clothed with spines than it, excepting
sometimes the few basal segments. All the five posterior pair of cirri
resemble each other more closely than is usual. In _D. Lowei_, the
segments of the posterior cirri bear the unusual number of eight pair of
main spines.
_Caudal Appendages._--Uni-articulate, spinose; in D. pellucida they are
twice as long as the pedicels of the sixth cirrus, but I could not
perceive in them any distinct articulations.
_Distribution._--Attached to crabs at Madeira, and off Borneo;
to sea-snakes in the Indian Ocean. The individuals of all the
species appear to be rare.
_General Remarks._--Four of the five species, forming this genus, though
certainly distinct, are closely allied. I have already shown, that
although the characters separating Lepas, Pæcilasma, and Dichelaspis are
not very important, yet if they be neglected these three natural little
groups must be confounded together. Dichelaspis is much more closely
united to Pæcilasma than to Lepas, and, as far as the more important
characters of the animal’s body are concerned, there is no important
difference between them. Consequently, I at first united Pæcilasma and
Dichelaspis, but the latter forms so natural a genus, and is so easily
distinguished externally, that I have thought it a pity to sacrifice it.
The carina, (which seems to afford better characters than the other
valves in Dichelaspis,) from generally running up between the terga and
in ending downwards, in three of the species, in a deeply notched disc
or fork, more resembles that in Lepas than in Pæcilasma; in the manner,
however, in which the imbedded disc, in _D. Warwickii_ and _D. Grayii_,
nearly cuts off the inside of the capitulum from the peduncle, there is
a resemblance to _Pæcilasma eburnea_. In the extent to which the valves
are separated from each other, in the bilobed form of the scuta, (the
two segments in Dichelaspis, perhaps, answering to the upper and lateral
projections in the scuta of _Conchoderma virgata_,) and in the basal
half of the scuta not descending to the base of the capitulum, there is
a considerable resemblance to Conchoderma; in both genera the adductor
muscle is attached under the umbones of the scuta; but the structure of
the mouth and cirri and caudal appendages shows that the affinity is not
stronger to Conchoderma than to Lepas. It appears at first probable,
that Dichelaspis would present a much closer affinity to _Pæcilasma
fissa_, in which, owing to the scuta being formed of two segments, there
are seven valves, than to any other species of that genus; but in _P.
fissa_ the primordial valve is triangular and is situated on the basal
segment, whereas, in Dichelaspis, it is elliptic and is seated between
the two segments, and is more in connection with the occludent than with
the basal segment; and this I cannot but think is an important
difference: in other respects, _P. fissa_ shows no more affinity to
Dichelaspis than do the other species of the genus. Finally, I may add
that Dichelaspis bears nearly the same relation to Pæcilasma, as
Conchoderma does to Lepas.
1. DICHELASPIS WARWICKII. Pl. II, figs. 6, 6 _a_, _b_.
OCTOLASMIS WARWICKII. _J. E. Gray._ Annals of Philosophy, vol. x,
p. 100, 1825; Spicilegia Zoologica. t. vi, fig. 16, 1830.
_D. scutorum segmento basali duplo latiore quam segmentum occludens:
tergorum parte inferiore paulò latiore quam occludens scutorum
segmentum._
Scuta, with the basal segment twice as wide as the occludent segment;
terga, with the lower part slightly wider than the occludent segment of
the scuta.
Mandibles, generally with four teeth.
Off Borneo, attached to a crab (Belcher): China Sea. British
Museum.
_General Appearance._--Capitulum much compressed, elongated, with the
valves not very close together, the carina being separated by a rather
wide space from the scuta and terga. Valves variable in shape, very thin
and translucent, covered by thin membrane, which, over the whole
capitulum, is studded with minute blunt points.
_Scuta._--Segments without internal teeth or an internal basal rim; the
occludent segment long, narrow, pointed, not quite flat, sometimes
slightly wider in the upper part; about one third of its own length
longer than the basal segment; occludent margin slightly arched; basal
segment about twice as wide as the occludent segment, triangular,
slightly convex; in young specimens (Pl. II, fig. 6 _b_), the carinal
margin of the basal segment is protuberant, and the occludent margin
hollowed out; in old specimens the occludent margin of the basal segment
is straight, and the carinal margin much hollowed out. In very young
specimens the basal segment is very small compared to the occludent.
_Terga_, variable in shape; flat, lower part wider than the occludent
segment of the scuta; occludent margin double, forming a considerable
rectangular projection, as in the terga of Lepas; scutal margin deeply
excised at a point corresponding with the apex of the scuta, a flat
tooth or projection being thus formed; there is sometimes a second tooth
(fig. 6 _b_) a little above the basal point. The terga, in the first
variety, somewhat resemble in shape the scuta of _Conchoderma aurita_.
_Carina_, much bowed, narrow, slightly concave within, (in the Borneo
specimen, rather wider and more concave,) extending up between the terga
for half their length, terminating downwards in a rectangularly
inflected, deeply imbedded, oblong, rather wide, flat disc, at its
extremity more or less deeply notched. This disc is externally smooth;
internally it sometimes has two divergent ridges on it; it extends
across about two-thirds of the base of the capitulum (fig. 6 _a_, as
seen from beneath, when the peduncle is cut off), to under the middle of
the basal segments of the scuta.
_Peduncle_, narrow, flattened; united to the capitulum some little way
below the scuta; about as long as the capitulum; the membrane of which
it is composed is thin, externally studded with bluntly conical beads of
yellowish chitine, of which the largest were 1/2000 of an inch in
diameter; on their internal surfaces these are furnished with a small
central, circular depression, apparently for a tubulus; the arrangement
of the beads varied in concentric zones. Similar conical points on the
capitulum have an internal concave surface about 1/3000 in diameter,
with a central circle 1/12000 in diameter, for the insertion, as I
believe, of a tubulus.
_Size._--The largest specimen had a capitulum a quarter of an inch long.
_Mouth._--Labrum highly bullate; crest with not very minute, blunt
teeth, which towards the middle lie closer and closer to each other, so
as to touch. Palpi rather small, with a few very long bristles at the
apex.
_Mandibles_, narrow, produced, with four teeth, and the inferior angle
tooth-like and acuminated; in one specimen, on one side of the mouth,
the mandible had only three teeth.
_Maxillæ_, small; at the upper angle there are two large spines and a
single small one, beneath which there is a deep notch, and beneath this
a straight but projecting edge, bearing a few moderately large and some
smaller spines. Outer maxillæ sparingly covered with bristles along the
inner margin.
_Cirri._--First pair far removed from the second pair, and not above
half their length; segments rather broad, with transverse rows of
bristles not very thickly crowded together; terminal segments very
obtuse, and furnished with thick spines. The segments of the three
posterior pair have each three or four pair of spines, with a few minute
spines scattered in an exterior, parallel, longitudinal row; dorsal
tufts, with four or five long spines. The second cirrus has its anterior
ramus not thicker, but rather shorter than the posterior ramus; the
former is only a little more thickly clothed with spines, owing to those
in the longitudinal lateral row being longer and more numerous, than is
the sixth pair of cirri. Bristles not serrated.
_Caudal Appendages_, narrow, thin, slightly curved, about half as long
as the pedicels of the sixth cirrus; in young specimens, the appendage
bore seven or eight pair of long bristles rectangularly projecting; in
some older specimens, there was a tuft of bristles on the summit, and
two other tufts on the sides.
I at first thought that the Borneo specimen was a distinct species, but
after careful comparison of the external and internal parts, the only
difference which I can detect is, that the terga are slightly larger,
and that the carina, to a more evident degree, is wider, more especially
in the middle and lower portions.
2. DICHELASPIS GRAYII. Pl. II, fig. 9.
_D. scutorum segmento basali angustiore quam segmentum occludens;
longitudine pæne dimidiâ: tergis bipenniformibus, margine crenato, spinâ
posticâ, manubrio angustiore quam occludens scutorum segmentum._
Scuta, with the basal segment narrower than the occludent segment, and
about half as long as it. Terga like a battle-axe, with the edge
crenated and a spike behind; the handle narrower than the occludent
segment of the scuta.
Mandibles with three teeth; cirri unknown.
Attached to the skin of a sea-snake, believed to have been the
_Hydeus_ or _Pelamis bicolor_, and therefore from the Tropical,
Indian or Pacific Oceans; associated with the _Conchoderma
Hunteri_; single specimen, in a very bad condition, in the Royal
College of Surgeons.
_General Appearance._--Capitulum much compressed, elongated, formed of
very thin membrane, with the valves forming round it a mere border.
Valves thin, imperfectly calcified, covered with membrane.
_Scuta_ formed of two narrow plates at very nearly right-angles to each
other, one extending along the occludent, and the other along the basal
margin; both become very narrow at the point of junction, and are there
not calcified, but are evidently continuous and form part of the same
valve; the basal segment is about half as long and narrower than the
occludent segment, flat and bluntly pointed at the end; occludent
segment slightly curled, and therefore the whole does not lie quite in
the same plane; narrow close to the umbo, with a very minute tooth on
the under side; apex rounded. In the upper part, the occludent segments
leave the membranous margin of the orifice, and run in near to the
terga, bending towards them at an angle of 45° with their lower part. I
was unable to distinguish the primordial valves.
_Terga._--These valves are of the most singular shape, resembling a
battle-axe, with a flat and rather broad handle; the upper part consists
of an axe, with a broad cutting crenated edge, behind which is a short
blunt spike. The spike and cutting edge together answer to the double
occludent margin of the tergum in Lepas. The whole valve is flat, thin,
and lies in the same plane; the carinal margin is nearly straight; the
scutal margin bulges out a little, and at a short distance above the
blunt basal point is suddenly narrowed in, making the lowermost portion
very narrow; the widest part of the handle of the battle-axe, is
narrower than the occludent segment of the scuta. The two spikes behind
the cutting and crenated edges of the two terga, are blunt and almost
touch each other; above their point of juncture, the membrane of the
orifice forms a slight central protuberance.