The generative system gives the chief interest to this genus. We here first meet with Males and Females distinct; and, within the limits of this same restricted genus, the far more wonderful fact of hermaphrodites, whose masculine efficiency is aided by one or two Complemental Males. The complemental and simple males closely resemble each other, as do the female and hermaphrodite forms; but under the two following species I enter into such full and minute details on these remarkable facts, that I will not here dilate on them. I may add that, at the end of the genus Scalpellum, I give a summary of the facts, and discuss the whole question. The penis (Pl. IV, fig. 9 _a_) in the hermaphrodite, _I. quadrivalvis_, is singular, from the length of its unarticulated support, and from the distinctness of the segments in the articulated portion. As ovigerous fræna occur in the usual place in _I. quadrivalvis_, though much smaller than in any other species, I have no doubt that they occur in _I. Cumingii_, although I failed in observing them. The glands on the margin, in _I. quadrivalvis_, are singular, from not being borne on a long, hair-like footstalk. _Affinities._--Ibla, though externally very different in appearance from Scalpellum, is more nearly related to that genus than to any other; in both genera some species have the sexes separate, the imperfect males being parasitic on the female, and other species are bisexual or hermaphrodite, but aided by parasitic complemental males. In Scalpellum, again, the oesophagus pursues a sinuous course, resembling that in Ibla, though it does not pass exteriorly to the adductor scutorum muscle. The disc of the prehensile antennæ of the larva, in both genera, has an unusual oblong form, like a mule’s hoof; there is also an affinity between the two genera in the size and form of the ova, in the prominent orifices of the olfactory cavities, and in the peduncle not being naked; though, in these two latter respects, in the structure of the cirri, and in the multiarticulate caudal appendages, there is an equal affinity to Pollicipes and Lithotrya. I have already shown that Alepas is likewise related to Ibla. 1. IBLA CUMINGII. Pl. IV, fig. 8. _I. (foem.) valvarum marginibus lateralibus, et superficie interiore, cæruleis: pedunculi spinis plerumque annulis cæruleo-fuscis._ Fem.--Valves coloured, along the lateral margins and on the upper interior surface, blue: spines on the peduncle, generally ringed with blueish-brown. Caudal appendages barely exceeding in length the pedicels of the sixth cirrus: rami of the first cirrus unequal in length by about two segments. Male,--with scarcely a vestige of a capitulum: maxillæ with fewer spines than in the female. _Hab._--Philippine Archipelago, Island of Guimavas; invariably attached to the peduncle of _Pollicipes mitella_, in groups of two or three together; Mus. Cuming. Tavoy, British Burmah Empire; Mus. A. Gould of Boston. FEMALE.