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. The _Rostrum_ is shorter, and internally more concave than the carina: I believe it projects more abruptly outwards than is represented in the figure. The _Peduncle_ commences some little way below the scuta: it is narrow and very short: it is finely villose: it is lined by delicate transverse striæ-less muscles, within which there are the usual stronger, longitudinal muscles. The base is flat and truncated. I examined, and carefully compared, the prehensile antennæ with those of the hermaphrodite, and found every part and every measurement the same. The full importance of this identity will hereafter be more fully insisted on. The antennæ are represented of their proper proportional size in fig. 3.