than to the parasites of Ibla. With respect to the parasites of the first three species of Scalpellum, they are in such an extraordinarily modified and embryonic condition, that they can hardly be compared with other Cirripedes; but certainly they do not approach the parasites of Ibla, more closely than the parasites of Scalpellum; and in the one important character of the antennæ, they are identical both with the parasitic and ordinary forms of Scalpellum. That two sets of parasites having closely similar habits, and belonging to the same sub-class, should be more closely related in their whole organisation to the animals to which they are respectively attached, than to each other, would, if the parasites were really distinct and independent creatures, be a most singular phenomenon; but on the view that they differ only sexually from the Cirripedes on which they are parasitic, this relationship is obviously what might have been expected. The two species of Ibla differ extremely little from each other, and so, as above remarked, do the two males. In Scalpellum the species differ more from each other, and so do the males. In this latter genus the species may be divided into two groups, the first containing _S. vulgare_, _S. ornatum_ and _S. rutilum_, characterised by not having a sub-carina, by the rostrum being small, by the constant presence of four pair of latera, and by the peculiar shape of the carinal latera; the second group is characterised by having a sub-carina and a large rostrum, and may be subdivided into two little groups; viz., _S. rostratum_ having four pairs of latera, and _S. Peronii_ and _villosum_ having only three pairs of latera: now the males, if classed by themselves, would inevitably be divided in exactly the same manner, namely, into two main groups,--the one including the closely similar, sack-formed males of _S. vulgare_, _ornatum_, and _rutilum_, the other the pedunculated males of _S. rostratum_, _Peronii_, and _villosum_; but this latter group would have to be subdivided into two little sub-groups, the one containing the three-valved male of _S. rostratum_, and the other the six-valved males of _S. Peronii_ and _S. villosum_. It should not, however, be overlooked, that the two main groups of parasites differ from each other, far more than do the two corresponding groups of species to which they are attached; and, on the other hand, that the parasitic males of _S. Peronii_ and _S. villosum_ resemble each other more closely, than do the two hermaphrodite forms;--but it is very difficult to weigh the value of the differences in the different parts of species.