Professor Steenstrup informs me that, from late observations, it appears that this animal always adheres to the shark’s body in pairs. I regret extremely that I have not been able to examine a pair: that the individual examined by me was bisexual, I can hardly doubt, though the male organs certainly were feebly developed; it appears probable, that the individual described by Lovén was likewise bisexual: but after the facts presently to be revealed regarding the sexes in Ibla and Scalpellum, it is quite possible that the male and female organs may be developed in inverse degrees in different and adjoining individuals. The genus Anelasma is, I think, properly placed between Alepas and Ibla. In several of its characters, such as the absence of calcareous valves, the broad blunt end of the peduncle, the spineless cirri, the small size of the trophi, and more especially the absence of transverse striæ in those muscles, which in mature cirripedes are thus furnished, we see that this genus is in some degree in an embryonic condition. _Genus_--IBLA. Pls. IV, V. IBLA. _Leach._ Zoolog. Journal. vol. ii, July, 1825. ANATIFA. _Cuvier._ Mem, pour servir, ... Mollusques, Art. Anatifa, 1817. TETRALASMIS. _Cuvier._ Regne Animal, 1830. (_Foem. et Herm._) _Valvæ 4, corneæ: pedunculus spinis corneis, persistentibus vestitus._ (Fem. and Herm.) Valves four, horny: peduncle clothed with persistent, horny spines. Body partly lodged within the peduncle; mandibles with three teeth; maxillæ with two obscure notches; outer maxillæ pointed; olfactory orifices prominent; caudal appendages multiarticulate. _Male and Complemented Male_, parasitic within the sack of the female or hermaphrodite; mouth and thorax seated on a long tapering peduncle, but not enclosed within a capitulum; mouth with normal trophi, but palpi small and almost rudimental; cirri rudimental, reduced to two pairs; penis reduced to a pore; caudal appendages rudimentary. Attached to fixed littoral objects: Eastern Hemisphere.