Chapter C: VIRGATA, var. OLFERSII (8)

_Muscles of the Peduncle._--These consist of the usual interior and longitudinal,--exterior and transverse--and oblique fasciæ; the former are unusually strong; downwards they are attached to the basal calcareous cup or disc, and upwards they extend all round to the lower curved margins of the valves. They are, as usual, without transverse striæ. Besides these, there are, (at least in _L. dorsalis_ and _L. Nicobarica_,) two little fans of striæ-less muscles, which occur in no other pedunculated cirripede; they are attached on each side of the central line of the carina, near its base; they extend transversely and a little upwards, and each fan converges to a point where the lower margins of the carina and terga touch; of these muscles, the upper fasciæ are the longest. Their action, I conceive, must be either to draw slightly together the basal points of the terga, and so serve to open their occludent margins, or to draw inwards the base of the carina: these muscles apparently first shadow forth the posterior or carinal, transversely-striated, opercular muscles of sessile cirripedes.

_Basal Calcareous Cup or Discs._--I have seen this part in all the species, except _L. Valentiana_, and in this it probably occurs, considering its very close alliance with _L. truncata_. The size, form, and conditions of the cup or disc varies infinitely according to the age, size, and position of the individual specimen. We will commence with a full-sized animal, which has ceased to burrow downwards into the rock, in which case the discs usually grow into a cup, and become largely developed. In _L. dorsalis_ alone, I have seen many specimens, so that the following description and remarks, though applicable I believe to all the species, are drawn up from that alone. The cup (Pl. VIII, fig. 1 _a´_, 1 _c´_) is hardly ever regular in outline, and is either slightly or very deeply concave; I have seen one, half an inch in diameter; it is formed of several thick layers of dirty white, translucent, calcareous matter, with sinuous margins; externally the surface is very irregular, and is coated by yellow membrane presently to be described. The innermost and last-formed layer sometimes covers the whole inside of the cup, and extends a little beyond its margin all round; but more generally it projects beyond only one side, leaving the other sides deserted. I have seen a _single_ new layer extending beyond the underlying old layers, as much as one sixth of an inch; and again I have seen a part of the cup, as much as a quarter of an inch in width, deserted and covered with serpulæ. So irregular, however, is the growth, that after a period an old deserted portion will occasionally be again covered by a new layer, though of course without organic adhesion. Again it sometimes happens that the last-formed layer, remaining central, is very much less than the older layers; in one such instance the innermost and last-formed layer (fig. 1 _a´_) had a diameter of only a quarter of that of the whole cup, in the middle of which it was placed; the cup thus tends to become filled up in the middle. The cup, in its fully developed condition, is seated at the very bottom of the cavity in the rock. From the aggregate thickness of the several component layers forming the cup, the old and mature animal rises a little in its burrow; for instance, the bottom of the cup in one specimen which I measured, was 4/10ths of an inch in thickness.

In a younger condition, before the animal has bored down to the full depth, and whilst the cavity is only of moderate diameter, the lower part of the peduncle, instead of being attached to the inside of a cup, adheres to small, irregular, nearly flat, calcareous discs, overlapping each other like tiles (figs. 1, 2 _a´_). They are placed one below the other, generally in a straight line, and are attached firmly to one side of the burrow. The discs are oval, or rounded, or irregular, and are commonly from 1/20th to 1/10th of an inch across: they usually form a quite straight ribbon, widening a little downwards: each little disc overlaps and extends beyond the one last formed, fully half its own diameter. I have seen one row of discs an inch in length, but the upper discs are always worn away by the friction of the calcified serrated scales on the peduncle. It is very important to observe that the lowest disc is not fixed, (as was the case with the cup,) at the very bottom of the burrow, but on one side, just above the bottom, which latter part is occupied by the blunt basal end of the peduncle.

In a valuable paper on _L. Nicobarica_, by Reinhardt, presently to be referred to, the disc is said to be attached on the carinal side (see fig. 2) of the peduncle; and this, I believe, is general. I have seen one instance in which, during the excavation of a new burrow, an old burrow was met with, and the row of discs turned down it, making, with their previous course, nearly a right-angle. In another similar instance, the discs, instead of turning down, became very large and broad, and so fairly formed a bridge across the old burrow (fig. 1),--becoming narrow again as soon as the animal recommenced burrowing into the solid rock. Sometimes, as it appears, the animal, whilst still small, from some unknown cause, stops burrowing downwards, and then a cup is formed at the bottom of the hole. As soon as the animal has got to its full depth, the burrow increases only in diameter, and during this process the linear row of discs is ground away and lost; a cup is then formed. The little discs can be deposited or formed only at each fresh exuviation; and as some of the burrows are above two inches in depth, and as on an average each disc does not extend beyond the underlying disc more than 1/15th of an inch, an animal which has bored two inches in depth, must have moulted at least thirty times. I may here remark that I have reason to believe, from some interesting observations made by Mr. W. Thompson, of Belfast, that some sessile cirripedes moult about every fortnight.

_Internal Structure of the Cup._--When the cup is dissolved in acid, each shelly layer is represented by a rather tough, pale-brown membrane, itself composed of numerous fine laminæ, which, under a one-eighth of an inch object glass, exhibit generally only the appearance of a mezzotinto drawing; but there often were layers of branching vessels, (like moss-agate,) less than the 1/10,000th of an inch in diameter, and of a darkish colour; these vessels are not articulated, but otherwise resemble the same peculiar structure in the valves of the capitulum. The exterior yellow membrane is marked, or rather composed of successive narrow rims, which, in fact, are the lines of termination of the laminæ of membrane, which in a calcified state form the cup itself. In most parts, both on the borders and under the centre of the cup, but not everywhere, there are imbedded in the yellow membrane, elongated, irregular, top-shaped masses of bright yellow chitine, each furnished with a tubulus, which penetrating the calcareous laminæ leads to the corium; the little apertures thus formed, are clearly visible in the layers of membrane, left after exposure to acid. In _L. Nicobarica_, the innermost shelly layer of the cup was punctured, like the surface of the shell in Chthamalus and many other sessile Cirripedes, by the internal orifices of these tubuli. The top-shaped masses often have star-shaped summits; and they differ in no essential respects from those on the lower part of the peduncle, excepting that they are quite imbedded in the membrane covering the under surface of the cup, whereas those on the peduncle project freely. I found these top-shaped bodies in the outer membrane of the cups in _L. dorsalis_, _L. cauta_, and _L. Rhodiopus_, which alone I was enabled to dissolve in acid; and I mention this fact, as indicating the probable presence of the more important star-headed projections on the lower parts of the peduncle in these same species. The basal calcareous cup resembles, in essential structure, the valves of the capitulum; the chief difference being that in the former there is a larger proportion of animal matter or membranous layers.

After the dissolution of the cups, in _L. dorsalis_ and _L. Rhodiopus_, I most distinctly traced the two cement-ducts; they included the usual darker chord of cellular matter; they were of rather small diameter, namely, 2/3000th of an inch. The two (in _L. dorsalis_) ran in a very irregular course, not parallel to each other, making the most abrupt bends. They passed through the membranous layers, (as seen after dissolution,) and running for short spaces parallel to the component laminæ, were attached to them. In their irregular course, these cement-ducts resemble those of _Pollicipes mitella_, but I could not perceive that any cement had been poured out at the abrupt bends. In one specimen of a basal cup, which I was enabled to examine whilst still attached to the rock, I found under the very centre, (and of course outside the yellow membrane,) a very small area of dark brown cement of the usual appearance. In several specimens of full-sized cups, I was not able to perceive any cement on the external surfaces of the upper and later-formed layers; hence I believe that the cup is cemented to the bottom of the hole only during the early stages of its formation; and this, considering its protected situation, would no doubt be sufficient to affix the animal. This probably accounts for the small size of the cement-ducts, and for the facility with which, as it appears, the cups can be removed in an unbroken condition from the rock. In the case, however, of the small, flat, calcareous discs, which are formed whilst the animal is burrowing into the rock, these are attached firmly to the sides of the holes, in the usual manner, by cement. In this cirripede it would be useless to look for the prehensile antennæ of the larva under the cup, for the animal, during the formation of the successive discs, must have travelled some distance from the spot on which the larva first attached itself.

The membrane of the peduncle is continuous with the yellow membrane coating the external surface of the cup; and this latter membrane is continuous with those delicate laminæ which, in a calcified condition, form the layers of the cup itself. In an exactly similar manner, in this and other cirripedes, the membrane of the peduncle, at the top, is continuous with that coating the valves, and is attached to the lower exterior edge of the last-formed layer of shell. When a new shelly layer is formed, both under the valves of the capitulum and inside the basal calcareous cup, it projects beyond the old layer, and is included within the old, as yet not moulted, membrane of the peduncle. Within the cup of _L. Nicobarica_ I found a lately-formed layer of shell, projecting 1/10th of an inch on one side of the cup, and by its protuberance distinguishable even through the old coat of the peduncle, which was nearly ready to be moulted. In an analogous manner, in the capitulum of _L. dorsalis_ and _L. truncata_, I have found a new peduncular membrane bearing the usual, but then sharp, calcified scales, attached to the lower projecting edge of the last-formed shelly layer, lying under the old peduncular membrane, which was attached to the penultimate layer of shell, and with its worn scales was just ready to be moulted.

The final cause of the moulting of the calcified scales, together with the membrane of the peduncle to which they are attached,--a case confined to Lithotrya,--I have scarcely any doubt is the reproduction of a succession of scales, sharply serrated for the purpose of enlarging the cavity in which the animal is lodged. The extreme thinness of the membrane of the peduncle has been noticed; this may be partly related to its protected condition, but partly, I think, to the necessity of its being formed in a very extensible condition; for the new coat, owing to the projection of the new shelly layers under the valves, and within the basal cup, is by so much shorter than the old peduncle, yet after exuviation it has to stretch to a greater length than the old membrane, to allow of the growth of the Cirripede. Owing to the thinness and fragility of this membrane, the basal attachment of the Cirripede is, no doubt, chiefly effected by the unusually strong longitudinal muscles; and the necessity of a surface of attachment for these muscles, stronger than the external membrane of the peduncle, probably is one of the final causes of the basal calcareous disc and cup, and likewise for the unusual manner in which the valves of the capitulum are locked together by folds and small roughened projections. The basal discs and cup, however, apparently serve for several other purposes, namely, for raising the animal a little in its burrow, (which is narrow and pointed at the bottom,) at that period of growth when it has ceased to burrow downwards, but still increases in diameter; also for carrying the animal, as over a bridge, across any pre-existing cavity in the rock; and lastly, perhaps, for removing lower down, in the intervals of exuviation, the point of attachment for the longitudinal peduncular muscles.

_Position of the animal in the rock, and its power of excavation._--A specimen of rock, two or three inches square, in Mr. Cuming’s possession, is full of Lithotryas; the cavities extend in every possible direction, and several were parallel, but with the animals in reversed positions; the same thing is apparent in some specimens of Mr. Stutchbury’s, and it was evident that the positions occupied by the animals were entirely due to chance. In Mr. Cuming’s specimen of rock, a considerable portion of the external surface is preserved, and here it can be seen that many of the specimens have their capitulums directed from the external surface directly inwards. These individuals, which were of full size, must have preyed on infusoria inhabiting the cavities of the porous, calcareous rock. On the other hand, I have seen some young specimens of _L. dorsalis_ with their valves not at all rubbed, and others of full size with uninjured Balani and corallines on the tips of the valves, and again a specimen of _L. truncata_ with minute pale-green sea-weed on the summit of the capitulum,--all which appearances induce me to believe that in these cases, the valves had projected freely beyond the cavity in which their peduncles were lodged. I may here also mention that in Mr. Cuming’s specimen, above alluded to, the basal cups of five specimens touched and adhered to each other; I was not able to make out whether there had originally existed separate burrows, as I think is most probable, and that the walls had been wholly worn away, or whether the five specimens had fixed themselves on one side of a large pre-existing, common cavity. Young specimens seem to burrow to the full depth, before nearly acquiring the diameter which they ultimately attain. I measured one burrow, 1.2 of an inch in depth, which, at its mouth or widest part, was only .17 in diameter.

The several species occur imbedded in soft calcareous rocks, in massive corals, and in the shells of mollusca and of cirripedes. It has been doubted by several naturalists, whether the basal calcareous cup at all belongs to the Lithotrya, but after the foregoing microscopical observations on its structure, it is useless to discuss this point. So again it has been doubted whether the cavity is formed by the cirripede itself; but there is so obvious a relation between the diameters of specimens of various sizes, and the holes occupied by them, that I can entertain no doubt on this head. The holes, moreover, are not quite cylindrical, but broadly oval, like the section of the animal. The simple fact, that in this genus alone each fresh shelly layer round the bases of the valves, and therefore at the widest part of the capitulum, are sharply toothed; and secondly, that in this genus alone a succession of sharply serrated scales, on the upper and widest part of the peduncle, are periodically formed at each exuviation; and that consequently the teeth on the valves and scales are sharp, and fit for wearing soft stone, at that very period when the animal has to increase in size, would alone render the view probable that the Lithotrya makes or at least enlarges the cavities in which it is imbedded.

Although it may be admitted that Lithotrya has the power of enlarging its cavity, how does it first bore down into the rock? It is quite certain that the basal cup is absolutely fixed, and that neither in form nor state of surface it is at all fitted for boring.[66] I was quite unable to answer the foregoing question, until seeing the admirable figures by Reinhardt[67], (Pl. VIII, figs. 2 and 2 _a´_) of _L. Nicobarica_, still attached in its cavity. Subsequently I obtained from Mr. Stutchbury several pieces of rock completely drilled with holes, many of small diameter, by _L. dorsalis_, and in these I found numerous instances of the linear rows of little discs, like those of _L. Nicobarica_, showing in the plainest manner, that each time a new disc is formed, that is, at each exuviation, the animal moves a short step downwards; and as the lowest of these little discs _in none of the burrows_ was placed at the very bottom, we see that the lowest point of the peduncle must be the wearing agent. In the peduncle of an individual of _L. dorsalis_, nearly ready to moult, I found, it may be remembered, beneath and round the basal disc, under the old membrane of the peduncle, a new membrane studded with calcified beads, but with the horny star-headed spines not yet developed, whilst on the old outer coat these latter had been worn down quite smooth, and the calcified beads worn entirely away. Here, then, we have an excellent rasping surface. With respect to the power of movement necessary for the boring action, the peduncle is amply furnished with transverse, oblique, and longitudinal striæ-less muscles,--the latter attached to the basal disc. In all the pedunculata, I have reason to believe that these muscles are in constant slight involuntary action. This being the case, I conceive that the small, blunt, spur-like portion of the peduncle, descending beneath the basal rim of the lowest disc, would inevitably partake slightly of the movements of the whole distended animal. As soon as the Lithotrya has reached that depth, which its instinct points out as most suitable to its habits, the discs are converted into an irregularly growing cup, and the animal then only increases in diameter, enlarging its cavity by the action of the serrated scales on the peduncle, and of the serrated lower edges of the valves of the capitulum. With respect to those reversed individuals attached with their capitulums downwards, I suppose that the larvæ had crept into some deep cavity, perhaps made originally by a Lithotrya, of which the rock in the specimen in question was quite full, and had there attached themselves. Finally, it appears that in Lithotrya the burrowing is simply a mechanical action; it is effected by each layer of shell in the basal attached discs overlapping, in a straight line, the last-formed layer,--by the membrane of the peduncle and the valves of the capitulum having excellent and often renewed rasping surfaces,--and, lastly, by the end of the peduncle (that is homologically the front of the head) thus roughened, extending beyond the surface of attachment, and possessing the power of slight movement.

[66] Mr. Hancock, in his admirable account of his burrowing
Cirripede, _Alcippe lampas_, (‘Annals of Nat. Hist.,’ Nov. 1849,
p. 313,) came to this conclusion regarding the cup of Lithotrya,
and hence was led to think that this genus did not form its own
burrows, but inhabited pre-existing cavities. I am much indebted
to this gentleman, who has been so eminently successful in his
researches on the boring powers of marine animals, for giving me
his opinion on several points connected with the present
discussion.

[67] I owe to the great kindness of Prof. Steenstrup the sight of
this Plate, published in the ‘Scientific Communications from the
Union of Natural History,’ Copenhagen, January 30, 1850, No. I.
Since this sheet has been set up in type, I have received from
Prof. Steenstrup the memoir, in Danish, belonging to the figures
in question; and the greater part of this has been translated to
me by the kindness of a friend. My account of the means of
burrowing is essentially the same as that published by Reinhardt;
but the moulting of the scales on the peduncle, the presence of
scales and of points of a different nature, the method of
attachment by cement, the conversion of the discs into a cup,
&c., seem not to have been known to this naturalist. Reinhardt
states that the points on the peduncle will scratch Iceland spar,
and that, apparently, they are formed of phosphate of lime: in
the case of the closely-allied _L. dorsalis_, I must believe that
the scales or beads on the peduncle are formed of carbonate of
lime, for they were quickly dissolved with effervescence in
acetic acid; and the star-headed points, which are subsequently
developed under the calcareous scales, appeared to me, under the
compound microscope, to be formed of a horn or chitine substance.
Reinhardt states that the basal point of the peduncle is arched a
little under the lowest disc, and there forms for itself a slight
furrow (as represented in the lateral view, Pl. VIII, fig. 2);
but in the burrows examined by me, this furrow or depression did
not really exist, the appearance resulting from the basal margin
of the lowest disc, projecting beyond the wall of the cavity by
the amount of its own slight thickness.

* * * * *

We will now proceed with our generic description.--

_Animal’s Body._--This, as already stated, is partially lodged within the peduncle. The prosoma is rather largely developed.

The _Mouth_ is placed at a moderate distance from the adductor muscle.

The _Labrum_ is moderately bullate, with a row of blunt bead-like teeth, mingled with fine bristles, on the crest, which in the middle part is generally somewhat flattened.

The _Palpi_ are blunt, and even squarely truncated at their ends; they are of large size, so that, if they had been half as large again, or even less, their tips would have met.

_Mandibles_ (Pl. X, fig. 2), with three nearly equal large teeth, and the inferior angle produced, broad, and strongly pectinated: in the interspaces between these teeth there are, in all the species, some very fine teeth or pectinations, which are seated a little on one side of the medial line. The mandibles are somewhat singular from the size of the transparent flexible apodemes (_a_ _a_) to which the muscles are attached; these are oval and constricted at their origins: in _L. dorsalis_ they are roughened with little points; in _L. cauta_ and _L. truncata_ they are large, of the same shape, but smooth.

_Maxillæ._--These are larger, compared to the mandibles, than is usual with pedunculated Cirripedes; they differ in shape in the different species, being either nearly straight on their edge, and notched or not (fig. 10), or notched with the inferior part forming a double prominence (fig. 12); the spines on the inferior angle, which is sometimes slightly produced, are always crowded together into a brush, and are finer than those on the upper parts. The apodemes are less straight than is usual, and at their origin take, in all the species, a rather abrupt bend; their extremity is enlarged into a little disc, which in _L. dorsalis_ is covered with strong points, but in the other species is, as usual, smooth.

_Outer Maxillæ._--The inner margin is slightly concave, and in _L. truncata_ alone, the bristles are hardly continuous, being interrupted in the middle part. The olfactory orifices are only very slightly prominent. The spines on all the trophi are more or less doubly serrated.

_Cirri._--The three posterior pair are elongated, with their anterior surfaces not at all protuberant. The segments bear from three to five pair of spines, with a row of three or four small intermediate spines; there are, as usual, some little lateral upper rim spines; the dorsal tufts contain some thick and thin spines mingled. _First_ cirrus is short, and placed not quite close to the second pair; the basal segments are broad and thickly paved with bristles. The _second_ pair is rather short compared with the _third_ pair; a varying number of the basal segments in both rami of both these cirri are protuberant, and are thickly paved with bristles; such segments are more numerous and are broader on the anterior rami than on the posterior rami. In _L. cauta_ alone, none of the basal segments in the posterior rami of the second and third cirri are thickly paved with bristles. The pedicels of the first three pair are irregularly covered with spines; those of the three posterior pair have the spines arranged in a regular double line. Most of the spines are doubly serrated.

_Caudal Appendages_ (Pl. X, fig. 23 and 24), multiarticulate, with thin elongated segments fringed with short spines; in length generally exceeding the pedicel of the sixth cirrus, and in _L. Nicobarica_ equalling half the entire length of this cirrus.

_Stomach_, destitute of cæca; oesophagus somewhat curled.

_Filamentary Appendages_, none.

_Ovaria_ filling up the peduncle and surrounding the sack, but not extending up to the bases of the scuta and terga; I saw the ova only in _L. truncata_; they were here oval and large, being nearly 9/400ths of an inch in length.

_Penis_, elongated; vesiculæ seminales extending into the prosoma. I noticed the ovigerous fræna only in _L. truncata_; here they were large, with an almost bilobed outline; the margin and whole lateral surface being covered with elongated cylinders, finely pointed, but not enlarged at their extremities, as are the glands observed in most of the other genera.

_Colours._--The posterior thoracic segments, the pedicels, the anterior and dorsal surfaces of the segments of the cirri, the caudal appendages, and the outer sides of the trophi are, in most of the species, more or less mottled with dark purple; parts of the interior surfaces of the valves in some of the species are coloured fine purple.

_Geographical Distribution._--The species are found all round the world in the tropical seas; this fact may have some connection with the presence of soft coral-reef limestone and of massive corals in these seas. The presence, however, of _L. cauta_ on the shores of New South Wales, shows that the genus is not strictly tropical.

_Affinities._--Lithotrya is a well-pronounced distinct genus; although there is a considerable difference in the shape of the valves between _L. dorsalis_ and _L. Valentiana_, at the opposite extremes of the genus, the strict uniformity of the internal characters shows that there are no grounds whatever for any generic separation; moreover, _L. Rhodiopus_ neatly blends together these extreme forms. Indeed it is not easy to imagine a better marked series of transitional forms, than those presented by the terga, in passing from _L. dorsalis_ through _L. Nicobarica_, _L. Rhodiopus_, and _L. truncata_, to _L. Valentiana_. Lithotrya has most affinity to _Scalpellum villosum_ or to _Pollicipes spinosus_ and _P. sertus_; though the affinity is far from close. In these two species of Pollicipes, we have seen that large irregular calcified spines are formed at the base of the peduncle, whereas in the other Pedunculata the scales or spines are formed exclusively round the upper margin of the peduncle. Lithotrya, as has been remarked by Sowerby and other authors, exhibits some affinity to the sessile Cirripedes, as shown by the calcareous basis,--by the manner in which the scuta and terga are locked together,--by the two little fans of muscle attached to near the basal points of the terga,--and perhaps by some of the characters of the trophi; nevertheless, this affinity is far from being well-marked, and I think is hardly so plain as in _Pollicipes mitella_.

1. LITHOTRYA DORSALIS. Pl. VIII, fig. 1 _a´_.

LITHOTRYA DORSALIS. _G.B. Sowerby._ Genera of Shells, April,
1822.

LEPAS DORSALIS. _Ellis._ Nat. Hist. Zoophytes, Tab. xv, fig. 5,
1786.

LITHOLEPAS DE MONT SERRAT. _De Blainville._ Dict. des Sc. Nat.,
Plate, fig. 5, 1824.

_L. scutis terga angustè obtegentibus: carinâ intùs concavâ: rostro, duorum aut trium squamarum subjacentium latitudinem æquante: lateribus, squamarum quinque subjacentium longitudinem æquantibus, superficie internâ angustè ellipticâ: pedunculi squamis superioribus verticillum secundum minus duplo superantibus._

Scuta, narrowly overlapping the terga: carina internally concave: rostrum as wide as two or three of the subjacent scales: latera with their internal surfaces narrowly elliptical, as long as five of the subjacent scales: upper scales of the peduncle less than twice as large as those in the second whorl.

Mandibles, with twice as many pectinations between the first and second main teeth, as between the second and third teeth. Maxillæ without a notch, edge nearly straight, and spines very numerous: caudal appendages exceeding, by half, the length of the pedicel of the sixth cirrus.

Barbadoes, West Indies; Venezuela; Honduras; imbedded in
limestone; Mus. Brit. Cuming and Stutchbury.

The state of preservation of the valves in different specimens varies greatly; generally only two or three, or even only the last-formed shelly layer, is preserved, the upper ones having scaled off; in a few young specimens, however, all the layers were perfect. The carina is generally better preserved than the other valves, and hence the upper part usually projects freely; in one specimen no less than ten zones of growth were preserved in the carina, whilst the other valves consisted of only three: the terga generally project rather more than the scuta. As each growth-layer is thick, if the scaling process had not taken place, all the valves would have projected greatly. The little teeth lie close together on the prominent serrated rims, on each zone of growth. The internal surfaces of the valves are roughened with small imbricated points. Exteriorly the valves are covered with yellow membrane, with rows, corresponding with each zone of growth, of very minute, yellow, horny spines, generally having their tips bent over, and so made hook-shaped. These spines are less than 1/600th of an inch in length.

_Scuta_, triangular; internally concave, with a large depression for the adductor muscle; there is the usual small roughened internal knob, or tooth, at the rostral angle of both the right and left hand valves. Tergal margin straight, overlapping about one third of the entire width of the terga.

_Terga_, irregularly oval, with the scutal margin straight; basal point blunt, with the two sides placed at about an angle of 45° to each other; the lower part of the carinal margin, immediately over the latera, (as seen internally,) is slightly hollowed out. Exteriorly, towards the bottom of the valve, from the overlapping of the scuta, of the latera, and of the carina, only a narrow rounded ridge is exposed, which runs down to the basal angle at about one third of the entire width of the valve, from the scutal margin. Internally the valve is slightly concave.

The _Carina_ slightly overlaps the terga; internally concave; generally with a large upper portion freely projecting; inwardly curved, without any central crest or ridge; valve nearly as wide as the middle part of the terga; inner growing or corium-covered surface, with its basal margin, protuberant and arched.

_Rostrum_ (Pl. VIII, fig. 1 _a´_, _a_, and greatly magnified 1 _b´_) very narrow; rarely more than two or three layers of growth are preserved; the sides are deeply sinuous, owing to each zone widening downwards; basal margin rounded; in width equalling about two and a half of the uppermost scales of the peduncle, and about half as wide as the latera.

_Latera_, small, placed obliquely, and parallel to the lower carinal margin of the terga; longer axis equal to five of the uppermost scales of the peduncle, and to nearly half the width of the base of the carina; growing surface (or a section made parallel to the growth-layers,) is narrow, elliptic, pointed at both ends, but the carinal half rather thicker than the scutal half.

The _Peduncle_ varies in length, generally about twice as long as the capitulum, in one specimen above thrice as long. The upper part as wide as the capitulum, the lower part sometimes much attenuated. The calcified scales in the uppermost whorl (Pl. VIII, fig. 1 _b´_) are only slightly larger than those in the second whorl; the scales in the succeeding three or four whorls, are considerably larger than those below, which latter very gradually decrease in size, till, low down on the peduncle, they are barely visible to the naked eye. In this lower part, they may be called calcareous beads; they stand some way apart from each other; they are nearly hemispherical, smooth, translucent, and furnished with a conical fang; some of the smallest were 1/325th and 1/400th of an inch in diameter. The upper scales vary somewhat in the outline, the most usual shape being sub-triangular, with the lower margin arched and protuberant; and this margin, in the two or three upper whorls, is crenated with teeth, which are conical and sharp, after exuviation, but soon become reduced to mere notches. The scales in the uppermost whorl are usually nearly quadrilateral; the imbedded portion, or fang of each scale, is, in all, produced into a blunt rounded point. The basal calcareous cup (fig. 1 _a´_ and 1 _c´_) is well developed, and is sometimes even half an inch in diameter. Before the cup is formed, there is a row of small, flat discs (fig. 1, and like those in fig. 2 _a´_) attached to the sides of the burrow: but a full account of these parts of the peduncle, and of the burrowing habits of this species, has been given under the generic description.

_Size and Colour._--Full average-sized specimens have a capitulum half an inch in width and height; the entire length, with the contracted peduncle, being about an inch and a half. Valves coloured dirty white, with the enveloping membrane, when preserved, yellow. The outer maxillæ, palpi, pedicels of the cirri, anterior faces of the segments, dorsal tufts, caudal appendages, and penis, dark purple. Thoracic segments brown. There is a purple spot between the bases of the first pair of cirri.

_Mouth._--Labrum considerably bullate, equalling about half the longitudinal diameter of the mouth; inferior part produced so as to separate the mouth some way from the adductor muscle; crest with a row of blunt teeth and hairs; central part depressed and flattened.

_Palpi_, rather large, separated from each other by only half their own length; bluntly pointed, thickly clothed with spines.

_Mandibles_ (Pl. X, fig. 2), with twice as many pectinations, namely 15, between the first and second main teeth, as between the second and third teeth, namely about 7; inferior angle strongly and coarsely pectinated; distance between the tips of the first and second main teeth, considerably less than between the tips of the second tooth and of the inferior angle; sides hirsute.

_Maxillæ_ (fig. 10), with the edge not quite straight, with the whole inferior part slightly projecting; spines very numerous, thirty or forty pairs; those close beneath the two upper great unequal spines, form a tuft and are rather thinner than the others, as are also those near the inferior angle; sides hirsute.

_Outer Maxillæ_, rather pointed, with the inner edge slightly concave, continuously and thickly clothed with short spines; spines on the outer edge long; there are also some minute, short, thinly scattered spines or points on the sides. Bristles on all the trophi doubly serrated.

_Cirri._--The first pair is placed at a small distance from the second. The segments in the three posterior pairs, support five pairs of very long spines, with a row of (I believe) four small intermediate spines; on the lateral upper edges, there are some short blunt spines; anterior faces of the segments not protuberant; the dorsal tufts consist of thick serrated, and of thin spines. The whole integument is hirsute with minute pectinated scales. Two or three of the basal segments in the sixth cirrus are confluent. _First cirrus_, anterior ramus rather shorter and thicker than the posterior ramus; basal segments thickly paved with serrated spines; in the posterior ramus, the six terminal segments are not paved with bristles. _Second cirrus_ has the seven basal segments of the anterior ramus very broad, and paved with bristles; the eight terminal segments having the usual structure; in the posterior ramus the three or four basal segments are similarly paved, but to a very much less degree, and the remaining thirteen have the usual structure. _Third cirrus_ has the six basal segments of the anterior ramus very broad and paved, and the fourteen terminal ones of the usual structure; in the posterior ramus, the three or four basal segments are similarly paved, but to a very much less degree, and the seventeen terminal ones have the usual structure. The pedicel of the first cirrus has very few spines; those of the second and third cirrus are thickly and irregularly clothed with spines; and those of the three posterior pair have a double row with intermediate small spines. On the antero-lateral faces of the pedicels of the second, third, and fourth pairs of cirri, there is an elongated white swelling or shield. Moreover, on the posterior thoracic segments, there are similar white-coloured swellings, with the membrane more plainly marked with scales than in other parts. The spines on the first three pairs of cirri are coarsely serrated.

_Caudal Appendages_ (Pl. X, fig. 23), with numerous tapering segments, almost equalling one and a half times the length of the pedicel of the sixth cirrus. Each segment is elongated and somewhat constricted in the middle, with its upper edge (fig. 24) crowned with short spines; in a full-sized specimen there were seventeen segments.

2. LITHOTRYA CAUTA. Pl. VIII, fig. 3.

_L. scutis terga amplè obtegentibus: carinâ intus concavâ: rostro squamarum subjacentium latitudinem vix æquante: lateribus, squamas subjacentes sesquitertio superantibus; superficie internâ latè ellipticâ: pedunculi squamis superioribus verticillum secundum pæne quadruplo superantibus._

Scuta largely overlapping the terga: carina internally concave: rostrum hardly as wide as one of the subjacent scales: latera with their internal surfaces broadly elliptical, as long as two and a half of the subjacent scales: upper scales of the peduncle nearly four times as large as those in the second whorl.

Mandibles with an equal number of pectinations between the first, second, and third main teeth: maxillæ notched, edge nearly straight: posterior rami of the second and third cirri, with their basal segments not paved with bristles: caudal appendages slightly exceeding in length the pedicels of the sixth cirrus.

New South Wales, Australia, imbedded in a Conia, (unique
specimen,) Mus. Stutchbury.

Valves thin, white, translucent; upper layers of growth well preserved, excepting on the terga. A large portion of the carina projected freely. The teeth on the projecting margins of the growth-layers are broad, blunt, and often stand rather distant from each other.

_Scuta_ (Pl. VIII, fig. 3 _a_), triangular, internally concave with no distinct pit for the adductor muscle. The scuta largely overlap the terga.

_Terga_ (fig. 3 _b_) approaching to rhomboidal; basal angle rectangular, almost central, and consequently the exterior longitudinal ridge, which is rounded, is likewise nearly central.

_Carina_, internally concave, with no trace of a central internal ridge in the upper free portion; the growing or corium-covered surface is transversely oval, and is as wide as the widest part of the terga.

_Rostrum_, exceedingly minute, enlarged at each zone of growth, not so wide as the immediately subjacent scale on the peduncle.

_Latera_ (fig. 3 _c_), in width equalling two and a half of the upper peduncular scales, or about one fourth or one fifth of the width of the carina; growing surface, (or a section parallel to the layers of growth,) broadly elliptic, pointed at both ends.

_Peduncle_, about twice as long as the capitulum; the scales of the uppermost whorl are quadrilateral (fig. 3 _d_), and nearly four times as large as those in the second whorl; these latter are about twice as large as those in the third whorl, which are very little larger than the small, almost equal-sized, equally distant, round beads scattered over the rest of the peduncle, down to the basal cup. All these scales are dentated, the upper rows most plainly and only on their basal margins; the lower little beads are very slightly crenated round their entire margins; they are mingled with star-headed spines (fig. 3 _e_) of yellow chitine. Basal calcareous discs thin, plainly marked exteriorly by concentric lines of growth, and covered by the usual yellow membrane, including the horny, spindle-shaped bodies.

_Size and Colours._--The whole specimen, including the peduncle, was only one fifth of an inch in length; the capitulum being 3/40ths of an inch in width. I do not know whether the specimen had attained its full size, but think this is probable, as a large-sized species would not have made its habitation in one of the valves of so small a shell as a Conia. Shell white, exterior membrane, where preserved, yellow, and bearing small spines. Thoracic segments, the lower segments of the second, third, and fourth cirri, all the segments of the first cirrus and the trophi, slightly mottled with darkish purple.

_Mouth._--The teeth or beads on the crest of the labrum are blunt, few, not very small, and equidistant.

_Palpi_, bluntly pointed.

_Mandibles_, with the three main teeth nearly equal in size; the pectinations are equal in number, namely, only three between the first and second, and the second and third main teeth; the inferior angle is coarsely pectinated, with one central spine much longer than the others; the distance between the tips of the first and second main teeth, equals that between the second tooth and the inferior angle.

_Maxillæ_, with the two upper spines very large; beneath them there are two small spines, and a considerable notch; the inferior part of the edge is nearly straight, bearing about thirteen pairs of spines, obscurely divided into two groups, the lower spines being smaller than the upper ones. The upper convex margin is hirsute with long hairs.

_Outer Maxillæ_, blunt, with the inner margin slightly concave; continuously, but thinly clothed with spines.

_Cirri._--The segments of the three posterior pairs bear four pairs of spines, with the usual intermediate fine spines; dorsal spines thin and thick mingled together. _First cirrus_, short, with the anterior ramus rather the thickest and shortest; all the segments thickly paved with bristles, except the two terminal segments, of which the ultimate one bears some serrated spines of most unusual length, namely, equalling within one segment the entire length of the ramus. I presume that these spines serve as feelers. _Second cirrus_; anterior ramus much thicker and considerably shorter than the posterior ramus; six basal segments paved with bristles, the two terminal segments having the usual structure; posterior ramus with all its nine segments on the usual structure. _Third cirrus_, longer, to a remarkable degree, than the second cirrus, with its anterior ramus having the four basal segments paved, and the seven terminal ones on the usual structure; posterior ramus with twelve segments, of which none are paved. The pedicels of the second and third cirri thickly and irregularly clothed with spines. The upper segments of the pedicels of all the cirri are unusually long.

_Caudal Appendages_, longer than the pedicels of the sixth cirrus, by barely one third of their own length. Segments much elongated, seven in number; I may add for comparison that each ramus of the sixth cirrus contained, in this specimen, sixteen or seventeen segments.

_General Remarks._--It is difficult to give obvious characters, (excepting the smallness of the rostrum compared with the scales on the peduncle,) by which this species can be externally discriminated from _L. dorsalis_, _L. Nicobarica_, and _L. Rhodiopus_; yet almost all the valves differ slightly in shape. In this species alone, (the peduncle of _L. Rhodiopus_ is not known,) the lower, microscopically minute, bead-like scales of the peduncle are crenated, though obscurely, all round. In the animal’s body, the diagnostic characters are strongly marked;--the long spines on the terminal segment of the first cirrus,--none of the segments in the posterior rami of the second and third cirri being thickened and paved with bristles,--the pectinations being equal in number between the main teeth of the mandibles,--are all characters exclusively confined to this species.

3. LITHOTRYA NICOBARICA. Pl. VIII, fig. 2.