If we look for analogies to the facts here given, we shall find them in the Lerneidæ already alluded to, but in these the males are not permanently attached to the females, only cling, I believe, to them voluntarily. The extraordinary case of the Hectocotyle, originally described as a worm parasitic on certain Cephalopoda, but now shown by Kölliker to be the male of the species to which it is attached, is perhaps more strictly parallel. So again in the entozoic worm, the Heteroura androphora the sexes cohere, but are essentially distinct: “this singular species, however,” according to Professor Owen,[49] “offers the transitional grade to that still more extraordinary Entozoon, the Syngamus trachealis, in which the male is organically blended by its caudal extremity with the female, immediately anterior to the slit-shaped aperture of the vulva. By this union a kind of hermaphroditism is produced; but the male apparatus is furnished with its own peculiar nutrient system; and an individual animal is constituted distinct in every respect, save in its terminal confluence with the body of the female. This condition of animal life, which was conceived by Hunter as within the circle of physiological possibilities, has hitherto been exemplified only in the single species of Entozoon, the discovery of the true nature of which, is due to the sagacity and patient research of Dr. C. Th. Von Siebold.” In Ibla, the males and females are not organically united, but only permanently and immovably attached to each other. We have in this genus the additional singularity of occasionally two males parasitic on one female.

I have used the term parasitic, which perhaps ought strictly to be confined to cases where one creature derives its nutriment from another, inasmuch as the male is invariably and permanently attached to and imbedded in the female,—from its being protected by her capitulum, so that its own capitulum is not developed—and from its feeding on minute animals infesting her sack. The male Ibla must seize its prey, guided probably by its well-developed olfactory organs, through the movement of its long, flexible body, furnished with muscles, and with the mouth seated on the summit. We have already seen one instance of a Cirripede, the Anelasma, obtaining its food without the aid of cirri, by means of its probosciformed, flexible mouth. The eye can serve only to announce to the male when the female opens her valves, allowing occasionally some minute prey to enter. In ordinary Cirripedes the penis is long, articulated, and capable of varied movements, I presume for the purpose of impregnating each separate ovum: the male Ibla has no such organ; and no doubt the whole body, furnished like the penis with longitudinal and transverse muscles, serves the same purpose! I may remark, that it seems surprising that so small a male should secrete sufficient semen to impregnate the ova of the female, but the ova are not nearly so numerous in Ibla as in most genera of Cirripedes; and the smallness of the males in some parasitic Crustacea has already been alluded to. The male must always be younger than the female, for the latter must first grow large enough for the larva of the male to crawl into her sack. Whether the male lives as long as the female I know not, but he certainly lives for a considerable period and increases in size, as shown by the depth to which the end of the peduncle is imbedded. Moreover we shall see, under the next species, that the male is metamorphosed from a larva, not one sixth of its own size.

In the male Ibla, abortion has been carried to an extraordinary and, I should think, almost unparalleled extent. Of the twenty-one segments believed to be normally present in every Crustacean, or of the seventeen known to be present in Cirripedes, the three anterior segments are here well developed, forming the peduncle: the mouth consists as usual of three small segments: the succeeding eight segments are represented by the rudimentary and functionless thorax, supporting only two pair of distorted, rudimentary and functionless cirri: the seven segments of the abdomen have disappeared, with the exception of the excessively minute caudal appendages; so that, of the twenty-one normal segments, fifteen are more or less aborted. The state of the cirri is curious, and may be compared to that of the anthers in a semi-double flower; for they are not simply rudimentary in size and function, but they are monstrous, and generally do not even correspond on opposite sides of the same individual. As males in other classes of the animal kingdom often retain some female characters, so here (though the case is not strictly analogous[50]) the male possesses the cementing apparatus, which homologically is part of an ovarian tube modified.

The individuals in every other genus (with the exception of Scalpellum), in the several families, in the three Orders of Cirripedia, are hermaphrodite or bisexual. Why, then, is Ibla unisexual; yet, becoming, in the most paradoxical manner, from its earliest youth, essentially bisexual? Would food have been deficient, and was the seizure of infusoria by another and differently constructed individual, necessary for the support of the male and female organs? The orifice of the sack of the female is unusually narrow; would the presence of testes and vesiculæ seminales have rendered her thorax and prosoma inconveniently thick? Seeing the analogous facts in the six, differently-constructed species of the allied genus Scalpellum, I infer there must be some profounder and more mysterious final cause.

2. Ibla quadrivalvis. Pl. IV, fig. 9.

I. (Herm.), valvis et pedunculi spinis sub-flavis: basali tergorum angulo, introrsùm spectanti, hebete, quia margo carinalis inferior longiùs quam margo scutalis prominet.

Hermaph.—Valves and spines on the peduncle yellowish: basal angle of the terga, viewed internally, blunt, owing to the lower carinal margin being more protuberant than the scutal margin.

Caudal appendages four times as long as the pedicels of the sixth cirrus: rami of the first cirrus unequal in length by about six segments.

Complemental Male, with a notched crest on the dorsal surface, forming a rudiment of a capitulum: maxillæ well furnished with spines.

HERMAPHRODITE.

All the external parts so closely resemble those of I. Cumingii, that it would be superfluous to describe more than the few points of difference. The horny substance of both scuta and terga is uniformly yellow; though in dryed specimens, from the underlying corium being seen through the valves, these generally have a tinge of blue.

The Scuta, viewed internally, are less elongated transversely; they have their basal margins slightly more hollowed out, and the fold on the upper free and horn-like portion rather deeper.

The Terga, viewed internally, have the apex of the growing or corium-covered surface higher relatively to the scuta than in I. Cumingii; and the basal angle is much broader, owing to the lower carinal margin being much more protuberant than the scutal margin. The spines on the peduncle are all yellowish-brown, and are rather longer than in I. Cumingii. I observed in three or four specimens, that the lowest part of the peduncle had become internally filled up with the usual, brown, transparent, laminated cement, cone within cone, so that this lower part was rendered rigid and stick-like; this latter effect, I apprehend, is the object gained by the formation of cement within the peduncle, of which I have not observed any other instance. The entire length of the largest specimen was one inch; some other specimens were only half this size.

The thorax and prosoma are of the same shape as in I. Cumingii, and in the largest specimen, about one tenth of an inch square; the prosoma, as in that species, is hairy. In the Mouth, all the parts are closely similar to those of I. Cumingii, but one third larger; the crest of the labrum is a little roughened with minute points: the palpi are squarer and blunter at their extremities: the mandibles have their second and third teeth nearly equal in size to the first, and they do not appear pectinated: the maxillæ have their spinose edge very nearly straight: the outer maxillæ are pointed. The olfactory orifices are similarly situated, and of similar shape; they are dark coloured.

Cirri.—These also are similar to those of I. Cumingii; the segments, however, of the three posterior cirri have each four pair of spines, placed very close together in a transverse direction. First cirrus has its two rami unequal in length by about six segments. The anterior rami of the second and third cirri are thicker, and more thickly clothed with spines, than the posterior rami, to perhaps a greater degree than in I. Cumingii. In the posterior cirri, the upper segments of the pedicels are nearly as long as the lower segments.

Caudal Appendages, four times as long as the pedicel of the sixth cirrus, and three fourths of the length of the rami of this same cirrus: segments thirty-two in number, and therefore as many as those forming the sixth cirrus: the upper segments are much thinner and longer than the basal segments; each furnished with a circle of short bristles; whole appendage excessively thin and tapering: the two closely approximate.

Colour.—From some well-preserved dryed specimens in Mr. Stutchbury’s possession, it appears that the sack, cirri and trophi, were dark blue, as in I. Cumingii; after being long kept in spirits, these parts become brown.

Generative System.—The penis (Pl. IV, fig. 9 a) is very singular in structure; it is of the ordinary length, but of small diameter; it tapers but little; it consists of a moveable articulated, and a fixed unarticulated portion; this latter is smooth, much flattened, not divided into segments, and projects straight out under the caudal appendages; it is about one third of the length of the entire penis; it corresponds with a part present in all Cirripedes, but here surprisingly elongated. The articulated portion consists of separate segments, twenty in number, quite as distinct as those of the cirri; each one is oblong, being longer by about a third part than broad; each has a few short bristles round its upper margin; the terminal segment has a circular brush of bristles. The vesiculæ seminales are easily seen, though they are narrow; they are slightly tortuous; they enter the prosoma, and lie on each side of the stomach; their outer case has a ringed structure, but is not fibrous; the contents in the best specimen consisted of a mass of spermatozoa, which I saw with perfect distinctness. The testes are unusually large and egg-shaped.

Ova, spherical, 5/400ths of an inch in diameter, united as usual into two ovigerous lamellæ. The ovigerous fræna are extraordinarily small, and might be very easily overlooked; their length, in a full-sized specimen, was only 7/400ths of an inch, and they projected only 2/400ths from the inner surface of the sack. The glands on their margin, to which the lamellæ adhere, are pointed oval, with an extremely short footstalk, and that rather thick; the entire length of gland and footstalk, being only 2/3000ths of an inch. The larvæ, in their first stage of development, offer the usual characters, and closely resemble those of Scalpellum; the probosciformed mouth, however, is remarkably prominent, and the limbs unusually thick.

Affinities.—This species most closely resembles I. Cumingii, and cannot be distinguished externally, except by the absence of the blue colour on the marginal and interior portions of the valves; and this can hardly be ascertained without separating and cleaning them, owing to the blueness of the underlying corium. Internally some slight differences may be perceived in the form of the valves. Considering these so slight differences, it is highly remarkable that this species should be hermaphrodite, whilst I. Cumingii is unisexual. There is a greater, though still slight, difference in the included animal’s body; the palpi in I. quadrivalvis are blunter, the mandibles smoother, the olfactory orifices darker-coloured; the rami of the first cirrus more unequal, the spines more numerous on the segments of the posterior cirri, and lastly and most conspicuously, the caudal appendages are very much longer relatively to the length of the sixth cirrus, than in Ibla Cumingii.

COMPLEMENTAL MALE.

I have examined one specimen of the hermaphrodite I. quadrivalvis, preserved in spirits from Kangaroo Island, and one dry from Adelaide, both places in South Australia, and four from an unknown locality, purchased from Mr. Sowerby; and within five out of these six specimens, males were attached. In one of them, two males of different ages were included, one adhering to the peduncle of the other: in I. Cumingii, also, it may be remembered, there was a case of two males parasitic on one female. I may add that I opened another quite young specimen, from Adelaide, not counted with the above, and it was without a male. The males in the five specimens were attached low down, at the rostral end, almost in a horizontal position, stretching across the bottom of the sack; one of them, however, was placed considerably on one side. One individual which I measured, was 16/100ths of an inch in length, and 5/100ths in width in the widest part, namely, about half down the peduncle. I may state, for the sake of comparison, that the hermaphrodite to which this individual was attached, was, including the peduncle and capitulum, one inch in length, that is, six times as long as the male, and one fifth of an inch in width, that is, four times as wide. The above measurements show that the male of this species is rather more than twice as large as that of I. Cumingii. In consequence of this greater size, I dissected, with the utmost care, the one specimen which was excellently preserved in spirits, and found every part, with a few exceptions, so exactly the same as in the male of I. Cumingii, only larger and more conspicuous, that it will be sufficient to indicate the few points of difference.

The most conspicuous difference is, that the oblique fold separating the thorax and peduncle is more plainly developed, projecting at the point corresponding to h in fig. 1, Pl. V, 8/1000ths of an inch; in the middle the fold is notched; it can be traced more easily than in I. Cumingii, running beneath and parallel to the basal edge of the mouth, to the ventral margin of the body. In the mouth there is hardly any difference; the maxillæ, however, have two notches even plainer than in the hermaphrodite I. quadrivalvis, or than in the male I. Cumingii, but the depth of such notches is always a variable character; there are also more spines on the edge in the male of the present species, than in I. Cumingii. Both mandibles and maxillæ in the male I. quadrivalvis, are larger than in the male I. Cumingii, to a greater degree than the larger proportional size of the body in the former will account for; and this, likewise, is the case with these same organs in the hermaphrodite I. quadrivalvis compared with the female I. Cumingii. The tubular olfactory orifices are situated in the same peculiar position as in the hermaphrodite, and as in both sexes of I. Cumingii: they are 1/500th of an inch in diameter, and about as thick as one of the lower segments in the rami of the sixth cirrus.

The thorax, as in the male of I. Cumingii, is quite rudimentary, and serves as a mere flap to protect the mouth. In the three specimens carefully examined, the posterior cirri had each only one ramus, whilst the anterior cirri generally had two: in one specimen, one of the rami in the anterior cirrus was formed of five segments, and the other ramus of three segments, both rami being supported on a uni-articulated pedicel; but on the opposite side of the same individual, the anterior cirrus was represented by a mere knob. The longer ramus of the anterior cirrus, in the best-developed individual, barely exceeded in length the mandibles measured along the line of the teeth! In one specimen between the bases of the posterior cirri, there were two perfectly distinct caudal appendages; these, like the cirri, are in a quite rudimentary condition; one was 5/1000ths of an inch in length, and consisted of three segments, the upper edges of which had short spines; the other was shorter, uni-articulated, but spinose. In a second specimen, these appendages were quite aborted. Close under them, on the inside or towards the mouth, (that is, in the normal position,) there was a rudimentary but quite distinct penis, with the apex projecting freely, and with the sides distinguishable from the ventral surface of the thorax, for the length of 1/1000th of an inch: the corium lining this little penis made the terminal orifice plainly visible. The vesiculæ seminales lie in the usual position, and are conspicuous; they are slightly tortuous, with their ends blunt: in the specimen so well preserved in spirits, they were filled with a mass of spermatozoa, perfectly distinct; and the whole cavity of the body was lined with globular and pear-shaped testes. Assuredly there was no vestige of ovarian tubes. From the greater size and excellent preservation of this specimen, which rendered the examination of the generative system so easy, I was able to examine the contents of the stomach, in which I found the delicate epithelial coat, separated as usual, and containing cellular matter, on which the animal had preyed, but the nature of which I was unable to make out. The anus was much plainer than in the male of I. Cumingii. I saw the eye distinctly. I could not distinguish the orifices of the acoustic (?) sacks; and I think I should have seen them, if they had existed.

Prehensile Antennæ.—I examined these in the larvæ presently to be mentioned, and therefore they were in better condition than in the mature animal when cemented. Their total length, measured along the outside, from the basal articulation to the end of the disc, is 32/6000ths or 33/6000ths of an inch—that is, one third longer than in I. Cumingii; whilst the hoof-like disc itself is 8/6000ths, or only 1/6000th of an inch longer than this same part in I. Cumingii: the apex of the disc is downy, or bears some excessively minute spines. The ultimate segment has its end irregularly rounded, with the spines obscurely divided into two groups, the outer group consisting of two or three longer and thinner spines, and the inner group of, as I believe, five rather shorter spines: the longer spines equal in length the whole ultimate segment. I could not perceive that they were plumose, as in many other genera. A single, rather thicker and long spine, pointing backwards, is attached to the under side of the disc, nearly opposite to the point where the ultimate segment is articulated on the upper convex surface. Another single, curved spine is attached on the outer side of the basal segment, near its distal end.

Development of the Male.—In the specimen before alluded to, which included two males, one of these was only the 30/1000ths of an inch in length, and therefore between one fifth and one sixth of the size of the mature male. It had, probably, undergone only one exuviation since its metamorphosis, for the larva is nearly as long, namely, 25/1000ths of an inch. In this young male, the mouth formed one third of the entire length: it was attached, not as in every other case to the sack of the hermaphrodite, but low down to the peduncle of the other male.

In the sack with these two males, there were certainly four, I believe five, larvæ, which in every main point of structure resembled the larvæ of other pedunculated Cirripedes. From the peculiar form of their prehensile antennæ, differing in no respect, except in the proportional lengths of the segments, from the same organ in the male I. Cumingii, I can feel no doubt that these were the larvæ of the male I. quadrivalvis;—for a moment’s reflection will show how excessively improbable it is, that several larvæ of some other Cirripede, and that a Cirripede intimately allied to the parasitic male Ibla, should have forced themselves, without any apparent object, into the sack of the hermaphrodite Ibla. The larvæ, though not yet attached, were on the point of attachment, so that the single eye of the mature animal could be distinctly seen, lying near to the two great compound eyes of the larva. We have also just seen, that one male quite recently here had undergone its metamorphosis. The larvæ are 25/1000ths of an inch in length, and rather more than 10/1000ths in width in the widest part: they are boat-shaped, the dorsal edge forming the keel of the boat; the anterior end is only a little blunter than the posterior end; the quasi-bivalve carapace is smooth. All the essential points of structure in the larvæ of other Cirripedes at this stage, could be distinctly here seen,—such as the two compound eyes, with the apodemes to which they are attached, and the two oblong sternal plates whence the apodemes spring,—the adductor muscle,—the six natatory legs, with long plumose spines,—the abdomen, with its three small segments and the caudal appendages,—the prehensile antennæ already described,—and, lastly, the two little (auditory?) sacks at the antero-sternal edges of the carapace, but not so near the anterior extremity as in Lepas. The four or five larvæ, after having undergone in the open sea the several preparatory metamorphoses common to the class, must have voluntarily entered the sack of the hermaphrodite: ultimately would they, on finding two males already attached there, have retired, and sought another individual less well provided; or would they all have remained, and so formed a polyandrous establishment, such as we shall presently see occurs sometimes in Scalpellum? This must remain quite uncertain.

In this same hermaphrodite specimen of I. quadrivalvis, the two ovigerous lamellæ contained some hundreds of larvæ in the first stage of development, which were liberated from their enveloping membranes by a touch of a needle: they were about the 16/1000ths of an inch in length, and presented all the usual characters of larvæ at this period. What a truly wonderful assemblage of beings of the same species, but how marvellously unlike in appearance, did this individual hermaphrodite present! We have the numerous, almost globular larvæ, with lateral horns to their carapaces, with their three pair of legs, single eye, probosciformed mouth and long tail:—we have the somewhat larger larvæ in the last stage of development, much compressed, boat-formed, with their two great compound eyes, curious prehensile antennæ, closed rudimentary mouth and six natatory legs so different from those in the first stage:—we have the two attached males, with their bodies reduced almost to a mouth placed on the summit of a peduncle, with a minute, apparently single eye shining through the integuments, without any carapace or capitulum, and with the thorax as well as the legs or cirri rudimentary and functionless:—lastly, we have the hermaphrodite, with all its complicated organisation, its thorax supporting six pairs of multi-articulated two-armed cirri, and its well-developed capitulum furnished with horny valves, surrounding this wonderful assemblage of beings. Unquestionably, without a rigid examination, these four forms would have been ranked in different families, if not orders, of the articulated kingdom.

Concluding Remarks.—If the creature which I have considered as the male of Ibla Cumingii be really so, and the evidence formerly given seems to me amply conclusive, then the animal just described, from its close affinity in every point of structure with the former, assuredly is the male of Ibla quadrivalvis. But feeling strongly how improbable it is, that an additional or complemental male should be associated with an hermaphrodite, I will make a few remarks on the only possible hypothesis, if my view be rejected,—namely, that the two parasites considered by me to be exclusively males, are not so, but are independent hermaphrodite Cirripedes, the female organs and ova (which, if present, would have been nearly mature, judging from the presence of spermatozoa in both species) having been overlooked by me in every specimen: and again, that in the animal described as the female I. Cumingii, I have, though minutely dissecting several specimens, and finding far smaller parts, such as the organs of sense and nervous system, entirely overlooked all the conspicuous male organs, though when I came to I. quadrivalvis, and naturally expected to find it likewise exclusively female, a single glance showed me the great probosciformed penis, and by the simplest dissection the vesiculæ seminales and testes were exhibited. Such an oversight is scarcely credible; but even if assumed, we have to believe in the extraordinary circumstance of the two parasites being species of an independent genus, not only the very next in alliance to the animals to which they are attached, but in certain most important points, namely, the organs of the mouth, actually deserving a place in the very same genus. Moreover, the two parasites differ from each other, not only in about the same slight degree, but in a corresponding manner, as do the two Iblas to which they are attached; thus the mouths of Ibla quadrivalvis and I. Cumingii are closely similar, (the difference being barely of specific value,) so are the mouths of the two parasites; but the parts are larger in the hermaphrodite I. quadrivalvis, than in I. Cumingii, so are they in the parasites. Again, the most conspicuous character in I. quadrivalvis, is the number of segments in the caudal appendages, far exceeding those in the other species of Ibla, as well as of every other pedunculated Cirripede, and the parasite of this species has articulated spinose appendages, far larger than the barely visible, non-articulated pair in I. Cumingii.

Considering the whole case, there seems no room to doubt the justness of the conclusion arrived at, under the former as well as under the present species, namely, that these little parasites are the males of the two species of Ibla to which they are attached;—wonderful though the fact be, that in one case, the male should pair with an hermaphrodite already provided with efficient male organs. It is to bring this fact prominently forward, that I have called such males, Complemental Males; as they seem to form the complement to the male organs in the hermaphrodite. We look in vain for any, as yet known, analogous facts in the animal kingdom. In the genus Scalpellum, however, next in alliance to Ibla, in which, consequently, if anywhere, we might expect to find such facts, they occur; and until these are fully considered, I hope the conclusions here arrived at, will not be summarily rejected. Although the existence of Hermaphrodites and Males within the limits of the same species, is a new fact amongst animals, it is far from rare in the Vegetable Kingdom: the male flowers, moreover, are sometimes in a rudimentary condition compared to the hermaphrodite flowers, exactly in the same manner as are the male Iblas. If the final cause of the existence of these Complemental Males be asked, no certain answer can be given; the vesiculæ seminales in the hermaphrodite of Ibla quadrivalvis, appeared to be of small diameter; but on the other hand, the ova to be impregnated are fewer than in most Cirripedes. No explanation, as we have seen, can be given of the much simpler case of the mere separation of the sexes in Ibla Cumingii: nor can any explanation, I believe, be given of the much more varied arrangement of the parts of fructification in plants of the Linnean class, Polygamia.

Genus—Scalpellum. Pls. V, VI.

(Herm. et Fœm.) Valvis 12 ad 15: lateribus verticilli inferioris quatuor vel sex, lineis incrementi plerumque convergentibus: sub-rostrum rarissime adest: pedunculo squamifero, rarissime nudo.

(Herm. and Fem.) Valves 12 to 15 in number: latera of the lower whorl, four or six, with their lines of growth generally directed towards each other: sub-rostrum very rarely present: peduncle squamiferous, most rarely naked.

Filamentary appendages, none: labrum, with the upper part highly bullate: trophi, various: olfactory orifices, more or less prominent: caudal appendages, uniarticulate and spinose, or none.

Males, parasitic at or near the orifice of the sack of the female or of the hermaphrodite: thorax enclosed within a capitulum, furnished with three or four rudimentary valves, or with six perfect valves: peduncle either short and distinct, or confounded with the capitulum: sometimes mouth and stomach absent, and cirri non-prehensile; sometimes mouth and cirri normal.

I have felt much doubt in limiting this genus: the six recent species which it contains, differ more from each other than do the species in the previous genera. Mr. Gray has proposed or adopted generic names for four of the species, and a fifth certainly has equal claims to this same rank. These genera have been founded almost exclusively on the number of the valves; and oddly enough, the numbers have generally been given wrongly, namely, in Scalpellum, Calantica, Thaliella, and Xiphidium. Scalpellum blends through S. villosum into Pollicipes; and this latter genus has an equal right with Scalpellum, to be divided into sub-genera, three in number. Hence, no less than eight genera might be made out of the twelve recent species of Scalpellum and Pollicipes, and their formation, in some degree, be justified; but, in my opinion, this inordinate multiplication of genera destroys the main advantages of classification. At one time, I even thought that it would be best to follow Lamarck, and keep the twelve recent species in one genus; but considering the number of fossil species, I believe the more prudent course has been followed, in retaining the two genera Scalpellum and Pollicipes; more especially as I can hardly doubt, that several other species will be hereafter discovered.

Having so lately described in the Memoirs of the Palæontographical Society, the fossil species, I will not here further allude to them, than to state, that out of the fifteen species therein described, S. magnum comes very close to the recent S. vulgare, and that several Eocene and Cretaceous species, such as S. quadratum, S. fossula, and S. maximum, are allied to S. rutilum and S. ornatum. Scalpellum villosum, a recent species, has stronger claims than any other species to be generically separated; and its habits, in not being attached to horny corallines, are also different, but the identity of its Complemental Male with that of S. Peronii, and its numerous points of resemblance in structure with the other species, have determined me not to separate it. Scalpellum Peronii, villosum, and rostratum, in having a sub-carina,—in the rostrum being pretty well developed,—and in the Complemental Male being pedunculated, and furnished with a functional mouth and prehensile cirri, may be separated from S. vulgare, ornatum and rutilum; but even between these two little groups, S. rostratum is in some respects intermediate, namely, in having three pairs of latera, and more especially in the rudimentary condition of the valves of its Complemental Male, and in the position in which the male is attached to the hermaphrodite. The three species in the second little group, namely, S. vulgare, S. ornatum, and S. rutilum, are more nearly allied to each other in all their characters, especially in the characters drawn from their Males, than are the other three species. S. ornatum and S. rutilum are considerably nearer to each other than any other two of the species. Upon the whole I conclude that the six species must be thrown either into five or into four genera (the first three species making one genus), or all into one genus, and this latter has appeared to me the preferable course. The separation even of Scalpellum and Pollicipes, as already stated, is hardly natural. The fact of these genera having existed from a remote epoch, and having given rise during successive periods to many species now extinct, is probably the cause that the few remaining species are so much more distinct from each other, than is common in the other genera of Lepadidæ. Whenever the structure of the whole capitulum in the fossil species is well known, and as soon as more species, recent and fossil, shall have been discovered, then probably the genus Scalpellum will have to be divided into several smaller genera.

Description.—The Capitulum is much compressed, and generally produced upwards; it is formed of from twelve to fifteen valves, which are rather thin, and with the exception of S. ornatum, almost entirely covered by membrane, bearing spines: the valves are seldom locked very closely together. A sub-rostrum exists only in S. villosum, which species leads on to Pollicipes; in S. vulgare the rostrum is rudimentary and hidden. The scuta, terga and carina, are much larger than the other valves: these five valves seem to differ essentially from the others in being at first developed under the form of the so-called primordial valves: the other valves commence by a small indistinct brown spot, very different from the hexagonal tissue of the primordial valves: I saw this very clearly in young specimens of S. vulgare. At first, the scuta, terga and carina, grow exclusively downwards (and permanently so in most fossil species), and therefore the growth of the scuta and carina is in an absolutely opposite direction to what it is in Lepas, Pæcilasma and Dichelaspis. After a short period the scuta are added to at their upper ends; the portion thus added, stands at a rather lower level, and projects in a rather different direction from the first-formed part of the valve, giving to it, in some respects, the appearance of having been broken and mended. This structure is common to S. vulgare, S. rostratum and S. Peronii. The upper Latera (except in S. villosum) grow in the same manner, namely, at first exclusively downwards, and then both upwards and downwards. The rostral and carinal latera (with the same exception of S. villosum) have their umbones seated laterally, at opposite ends of the capitulum,—the umbones of the rostral latera being close to the rostrum, and those of the carinal pair close to the carina, and consequently their chief growth is directed towards each other. The carina in all the species, except S. villosum, is either bowed or angularly bent; in the latter case the lower half is parallel to the peduncle, and the upper half, extending far up between the terga, is parallel to their longer axes. In some of the species the carina is added to almost equally at both ends; in S. ornatum it grows but little at the upper end, and to a varying degree in different individuals according to their age; in S. rutilum the umbo is at the apex, and there is consequently no upward growth; lastly, in S. villosum the carina widening much from the apex to the basal margin, grows exclusively downwards, and a portion of the apex projects freely,—characters all common to the carina in the genus Pollicipes. The upper latera occur in all the species; in the lower whorl there are either two or three pair of latera, in the former case the infra-median pair being absent. The latera differ considerably in shape in the different species.

The Peduncle is generally rather short, and, with the exception of S. Peronii, is covered with calcified scales. These scales are generally small, and placed symmetrically in close whorls, in an imbricated order, with each scale corresponding to the interspace between two scales in the whorls above and below. In S. ornatum, the scales are so wide, transversely, that there are only four in each whorl. In S. villosum, the scales are spindle-shaped and arranged somewhat irregularly in transverse rows, not very near to each other. New calcareous scales originate only round the top of the peduncle, and they continue to grow only in the few upper whorls; and as the peduncle itself continues to increase in diameter by the formation of new inner membranous layers and the disintegration of the old outer layers, the calcareous scales come in the lower part of the peduncle to stand further and further apart. In the earliest stage of growth there are no calcareous scales on the peduncle in S. vulgare; they first appear under the carina. Spines are articulated in great numbers on the surface of the peduncle in S. vulgare, S. Peronii, and S. villosum, and very short ones on that of S. rostratum.

Attachment.—All the species, except S. villosum, are attached to horny corallines: the singular means of attachment in S. vulgare will be described under that species, and is probably common to several of the other species. The larva in most, or in all cases, when it proceeds to attach itself, clings head downwards to the branch, and hence the capitulum comes to be placed upwards, with its orifice fronting the branch and the carina outwards. The sucking disc of the prehensile antennæ of the larva, in the five species examined, was a little pointed, and in shape resembled the hinder hoof of a mule: this may perhaps be accounted for by the narrowness of the branches of the corallines, to which it has to adhere: a large circular disc, as in Lepas, would have been worse than useless: the ultimate segment in most or all the species, has on its inner side (the segment being supposed to be extended straight forward) a notch or step, bearing, I believe, two spines.

Size and Colour.—Some of the species attain a medium size, others are small. The valves are generally clouded red or pink, but sometimes white.

Mouth.—The various parts vary far more than in any genus hitherto described. The labrum is highly bullate, with the upper part forming a rounded overhanging projection, and with the lower part much produced, so that the mouth is placed far from the adductor scutorum muscle, and consequently the orifice is directed more towards the ventral surface of the thorax than in most other Cirripedes: on the crest of the labrum there are some very small teeth in several of the species, but not in all. The mandibles have either three or four main teeth, generally with either one or two small teeth intermediate between the first and second large teeth, and in the case of S. Peronii, with small teeth between all the larger ones. The maxillæ have their edges furnished with many spines, and are either straight or have the inferior part prominent and step-formed. The outer maxillæ have the spines on their inner edges either continuous or divided into two groups, of which latter structure we have not hitherto had any very well characterised example. The olfactory orifices are either highly or moderately protuberant.

In most of the species the prosoma is little developed, and the first cirrus is placed far from the second. The Cirri are generally but little curled, and have elongated segments, with long, generally serrated spines: the first cirrus varies in proportional length; the second and third cirri have both their rami more thickly clothed with spines than are the three posterior cirri, the spines being generally arranged in three or four longitudinal rows: the cirri, however, of S. villosum in all respects resemble closely the cirri of Pollicipes sertus and P. spinosus.

The Caudal Appendages are uniarticulate, small, and clothed with spines: in S. villosum, however, differently from in all other allied forms, there are no appendages.

The Stomach, in those species which I opened, is destitute of cæca. There are no filamentary appendages.

Generative System. The ova are nearly spherical, and remarkably large, as was stated to be the case in the introductory discussion, in which the larva of S. vulgare, in the first stage of development, was described: the ovigerous fræna are small. The testes are large, but the vesiculæ seminales in some of the species extraordinarily small. Scalpellum ornatum, and perhaps S. rutilum, are unisexual; the other species are hermaphrodite, but most or at least some of the individuals, are furnished with Complemental Males. These latter are fully described under each species, so I will here only remark, that S. ornatum, which alone (excepting perhaps S. rutilum) is unisexual, has less claim than the other species to be generically separated: we have seen also, in Ibla, that similar sexual differences occur in two most closely allied species. It is very singular how much more some of the Males and Complemental Males in Scalpellum differ from each other, than do the female and hermaphrodite forms; this seems due to the different stages of embryonic development at which the males have been arrested. In the males, however, of S. rostratum, S. Peronii, and S. villosum, compared one with another, but not with the males of the other species, the parts of the mouth and apparently the cirri, resemble each other more closely, than do the same organs in the hermaphrodites. At the end of this genus I shall give a summary on the highly remarkable sexual relations both in Scalpellum and Ibla.