_Cirri._--These consisted, in the one specimen, of merely small fragments. The segments of the posterior cirri are elongated, not protuberant, and support, I believe, five pair of non-serrated spines, and an exterior row of very minute spines: dorsal spines fine and long. Either the second or third cirri, or probably both, are remarkable for having the whole of one side of each segment covered with irregular rows of long spines. Moreover, in the upper segments of these same cirri, between each separate dorsal tuft, there is placed one or two long bristles. The first cirrus appears to have had very broad segments, and these are singular from the spines in the dorsal rows, being extremely long. In some of the cirri, several of the basal segments are soldered together. _Caudal Appendages_, lost. From the state of the specimen, it was quite impossible to ascertain whether the individual here described was an hermaphrodite or female; from the analogy of its nearest congener, _S. ornatum_, the latter is the most probable; but the genus Ibla shows how the sexes may differ in the most closely-allied forms. _Affinities._--From the hollows on the under sides of the scuta, for the lodgment of the males; from the umbones of the scuta and of the carina being situated on the apices of these valves; and from all the characters of the mouth, _S. rutilum_ is much more closely allied to _S. ornatum_ than to any other species. MALE, OR COMPLEMENTAL MALE. In the concavity or hollow above the depression for the adductor muscle (Pl. VI, fig. 2 _a´_), I found males, but in so extremely decayed a condition, that they could hardly be examined. On one side, however, I distinctly saw the larval prehensile antennæ, with pointed, hoof-like discs; and part of the thorax, with its small limbs and long spines, as in _S. vulgare_ or _S. ornatum_. I also saw clearly the eye. The four calcified beads or rudimentary valves, I believe, were present; but in removing the specimen, the whole fell to pieces and was lost. The outer integument was covered with rather thick, very minute bristles, each about, 2/10,000th of an inch in length, and therefore only half the length of those on the complemental males of _S. vulgare_. The cavities for the males are not formed, as in _S. ornatum_, by the thickening of the internal surface of the valve round a defined space, but by the scutum being externally convex and internally concave down the middle, hollows being thus produced both for the lodgment of the males and for the attachment of the adductor muscle. These hollows are separated from each other by a slight transverse ridge. I do not know at which point of the margin of the valve, the orifice of the male is situated, but I presume close under the apex. In this species, as in _S. ornatum_, there can be no question that the scuta of the female are specially modified by their own growth for the reception of the males. It must be added