Outer Maxillæ, large and triangular. The inner margin is slightly concave, and continuously covered with short spines. The outer margin is bilobed, as in S. vulgare, with the basal part supporting a great tuft of long bristles, of which the greater number turn outwards, and almost cover the olfactory orifices. The latter are slightly prominent, placed some way apart from each other, with the above-mentioned tufts of bristles between them. All the spines of the trophi are in some degree doubly serrated.

Cirri.—The first pair is seated rather far from the second pair, and the prosoma being little developed, the shape of the body nearly resembles that of S. vulgare. The posterior cirri are elongated, very little curled, with the segments much flattened, not at all protuberant, bearing from five to seven pair of long serrated spines, with a few small spines in an exterior row; between each pair there is a very minute tuft of small bristles; the upper lateral rim of each segment is toothed with small spines; spines of the dorsal tufts, long, serrated. First pair, elongated, having numerous segments, namely, seventeen, whilst the sixth pair in the same individual had only twenty-one segments; rami nearly equal; segments short, nearly cylindrical, thickly clothed with long serrated spines. The second and third pair are nearly equal in length; they have their anterior rami slightly thicker than their posterior rami, both being much more thickly clothed with spines, than are the three posterior pair of cirri. Pedicels, rather short, with their inner edges not forming a projection, as in S. vulgare.

Caudal Appendages ([Pl. X], [fig. 20]), uni-articulate, flat, rounded at their ends and moderately long; clothed most thickly, like brushes, with very fine bristles, which latter are serrated, and are longer than the appendages themselves.

Penis, of small size, narrow, pointed, and thickly clothed with delicate hairs; in length equalling only one fourth of the sixth cirrus.

Ovigerous Fræna, small, semicircular; entire edge thickly covered with glands. Ovarian tubes, within the peduncle, fully developed as usual.

Affinities.—This species differs from all the others in the absence of calcareous scales on the peduncle; but it has no other character which at all justifies its generic separation. In the shape of the scuta and carina it comes nearest to S. vulgare. Taking all the characters together, it is scarcely possible to say to which of the other species it is most closely allied, having close affinities with all. In the entire structure, however, of the Complemental Male, immediately to be described, this species certainly comes nearer to S. villosum than to any other species. I may add, that in S. villosum the latera are almost rudimentary, and therefore tend to disappear, whereas in S. Peronii it is the calcareous scales on the peduncle which have actually disappeared.

COMPLEMENTAL MALE. [Pl. VI], [fig. 3.]

I examined, owing to the great kindness of Mr. Cunning, six dry specimens of the hermaphrodite S. Peronii, from Swan River, and one in spirits from another locality, in the British Museum. Out of these seven specimens, only three appeared to have had parasites attached to them, and these I infer, from reasons to be more fully given at the end of the genus, are Complemental Males. One of the three specimens, however, had two males close together. These parasites were firmly cemented to the integument of the hermaphrodite, in a fold, in a central line between the scuta, a little below (the animal being in the position in which it is figured) the adductor scutorum muscle, and therefore some way below the umbones of these valves. When the scuta are closed, the parasites, from their small size, are enclosed and protected. In every detail of structure, they are obviously pedunculated Cirripedia.

The Capitulum ([Pl. VI], [fig. 3]) has six valves; namely, a pair of scuta and of terga, a carina, and a rostrum, all united by finely-villose membrane, furnished near the orifice with some much longer and thicker spines. The capitulum is truncated in a remarkable manner, the orifice not being, as in the hermaphrodite, in the same line with the peduncle, but almost transverse to it, and therefore almost parallel to the surface of attachment. The largest specimen measured transversely, through the scuta and terga, was 30/1000ths of an inch in breadth; another was only 26/1000ths to 27/1000ths: this latter specimen, measured longitudinally, from the base of the carina to the tips of the terga, was 15/1000ths of an inch. A scutum of the largest specimen was 17/1000ths in length. The scuta and terga are broadly oval, with the primordial valves very plain at their upper ends. I may here mention, that in a central line between the scuta, I observed the apparently single, minute, black eye, as in ordinary Cirripedia.

The Carina is straight, triangular, and internally slightly concave; its basal margin descends far below the basal points of the terga.