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 that, as it was not possible to ascertain whether the ordinary form of S. rutilum was hermaphrodite or female, so it must remain doubtful whether the parasites are males or complemental males; but the former, I think, is most probable.

[† † SUB-CARINÂ PRESENTE.]

4. Scalpellum rostratum. [Pl. VI], [fig. 7.]

S. (Herm.) valvis 15: rostro permagno: laterum paribus quatuor: pari superiore pentagono.

(Herm.) Capitulum with 15 valves: rostrum very large: four pair of latera; upper latera pentagonal.

Mandibles with four teeth; maxillæ with the inferior angle prominent.

Complemental Male, attached between the mouth and adductor scutorum muscle; pedunculated; capitulum bearing a pair of elongated scuta and a rudimentary carina; mouth and cirri prehensile.