_General Remarks and Affinities._--The first five species form a most natural genus; they are often sufficiently difficult to be distinguished, owing to their great variability. The sixth species (_L. fascicularis_) differs to a slight extent in many respects from the other species, and has considerable claims to be generically separated, as has been proposed by Mr. Gray, under the name of Dosima; but as it is identical in structure in all the more essential parts, I have not thought fit to separate it. As far as external characters go, some of the species of Pæcilasma have not stronger claims, than has _L. fascicularis_, to be generically separated; and I at first retained them altogether, but in drawing up this generic description, I found scarcely a single observation applicable to both halves of the genus; hence I was led to separate Lepas and Pæcilasma. If I had retained these two genera together, I should have had, also, to include the species of Dichelaspis and Oxynaspis; and even Scalpellum would have been separable only by the number of its valves; this would obviously have been highly inconvenient. Although some of the species of Pæcilasma so closely resemble externally the species of Lepas, yet if we consider their entire structure, we shall find that they are sufficiently distinct; as indirect evidence of this, I may remark that Conchoderma (as defined in this volume), includes two genera of most authors, and yet certainly comes, if judged by its whole organisation, nearer to Lepas than does Pæcilasma. 1. LEPAS ANATIFERA. Tab. I. fig. 1. (_var._) L. ANATIFERA. _Linnæus._ Systema Naturæ, 1767. ANATIFA vel ANATIFERA vel PENTALASMIS lævis[24], plerumque auctorum. ---- ENGONATA (!).[25] _Conrad._ Journal Acad. Nat. Sc. Philadelphia, vol. vii, 1837, p. 262, Pl. xx, fig. 15. ---- DENTATA (var.) _Brugière._ Encyclop. Meth. (des Vers), 1789. PENTALASMIS DENTATUS (var.) _Brown._ Illust. Conch., Pl. lii, fig. 5. ANATIFA . . . . . _Martin St. Ange._ Mem. sur l’organisation des Cirripedes, 1835. [24] As this, though the commonest species, has never been defined, I give only a few synonyms and references, it being quite impossible to distinguish, in any published description, this species from _A. Hillii_ of Leach; this latter species I recognise under this name only from having authentic specimens from the British Museum, as Leach overlooked every one of the real diagnostic characters. [25] I have used, in conformity with botanists, the mark of interjection, to show that I have seen an authentic specimen. _L. valvis aut lævibus aut delicate striatis: è duobus scutis, dextro solùm dente interno umbonali instructo; pedunculi parte superiore fuscâ._ Valves smooth, or delicately striated. Right-hand scutum alone furnished with an internal umbonal tooth: uppermost part of peduncle dark-coloured. Filaments, two on each side.