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.