_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 ma.s.sive corals in these seas. The presence, however, of _L. cauta_ on the sh.o.r.es of New South Wales, shows that the genus is not strictly tropical.
_Affinities._--Lithotrya is a well-p.r.o.nounced 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 pa.s.sing 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 Sh.e.l.ls, 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 anguste obtegentibus: carina intus concava: rostro, duorum aut trium squamarum subjacentium lat.i.tudinem aequante: lateribus, squamarum quinque subjacentium longitudinem aequantibus, superficie interna anguste elliptica: 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. Maxillae 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. c.u.ming 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 sh.e.l.ly 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, h.o.r.n.y 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 k.n.o.b, 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 maxillae, palpi, pedicels of the cirri, anterior faces of the segments, dorsal tufts, caudal appendages, and p.e.n.i.s, 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 coa.r.s.ely 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.
_Maxillae_ (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 Maxillae_, 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 coa.r.s.ely 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 ample obtegentibus: carina intus concava: rostro squamarum subjacentium lat.i.tudinem vix aequante: lateribus, squamas subjacentes sesquitertio superantibus; superficie interna late elliptica: pedunculi squamis superioribus verticillum secundum paene 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: maxillae 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 h.o.r.n.y, 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 sh.e.l.l as a Conia. Sh.e.l.l 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 coa.r.s.ely 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.
_Maxillae_, 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 Maxillae_, blunt, with the inner margin slightly concave; continuously, but thinly clothed with spines.