Considering the whole case, there seems no room to doubt the justness of the conclusion arrived at, under the former as well as under the present species, namely, that these little parasites are the males of the two species of Ibla to which they are attached;--wonderful though the fact be, that in one case, the male should pair with an hermaphrodite already provided with efficient male organs. It is to bring this fact prominently forward, that I have called such males, Complemental Males; as they seem to form the complement to the male organs in the hermaphrodite. We look in vain for any, as yet known, analogous facts in the animal kingdom. In the genus Scalpellum, however, next in alliance to Ibla, in which, consequently, if anywhere, we might expect to find such facts, they occur; and until these are fully considered, I hope the conclusions here arrived at, will not be summarily rejected. Although the existence of Hermaphrodites and Males within the limits of the same species, is a new fact amongst animals, it is far from rare in the Vegetable Kingdom: the male flowers, moreover, are sometimes in a rudimentary condition compared to the hermaphrodite flowers, exactly in the same manner as are the male Iblas. If the final cause of the existence of these Complemental Males be asked, no certain answer can be given; the vesiculæ seminales in the hermaphrodite of _Ibla quadrivalvis_, appeared to be of small diameter; but on the other hand, the ova to be impregnated are fewer than in most Cirripedes. No explanation, as we have seen, can be given of the much simpler case of the mere separation of the sexes in _Ibla Cumingii_: nor can any explanation, I believe, be given of the much more varied arrangement of the parts of fructification in plants of the Linnean class, Polygamia. _Genus_--SCALPELLUM. Pls. V, VI. SCALPELLUM. _Leach._ Journ. de Physique, t. lxxxv, July, 1817. LEPAS. _Linn._ Systema Naturæ, 1767. POLLICIPES. _Lamarck._ Animaux sans Vertebres, 1818. POLYLEPAS. _De Blainville._ Dict. des Sc. Nat., 1824. SMILIUM (pars generis). _Leach._ Zoolog. Journal, vol. 2, July, 1825. CALANTICA (pars generis). _J. E. Gray._ Annals of Philosophy, vol. x, (new series,) Aug. 1825. THALIELLA (pars generis). _J. E. Gray._ Proc. Zoolog. Soc., 1848. ANATIFA. _Quoy_ et _Gaimard_. Voyage de l’Astrolabe, 1826-34. XIPHIDIUM (pars generis). _Dixon._ Geology of Suffolk, 1850. (_Herm. et Foem._) _Valvis 12 ad 15: lateribus verticilli inferioris quatuor vel sex, lineis incrementi plerumque convergentibus: sub-rostrum rarissime adest: pedunculo squamifero, rarissime nudo._ (Herm. and Fem.) Valves 12 to 15 in number: latera of the lower whorl, four or six, with their lines of growth generally directed towards each other: sub-rostrum very rarely present: peduncle squamiferous, most rarely naked.