from the reasons formerly assigned, form a new genus,
nearer to Ibla than to the parasites of Scalpellum: so, again, the
assumed males of the three latter species of Scalpellum would form two
new genera, both of which would be more closely allied to Scalpellum,
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 antennae, 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
s