_Geological History._--Having treated this subject at length, in the volume of the Palæontographical Society for 1851, I will not here enter on it: I will only remark, that the Lepadidæ or Pedunculated Cirripedes are much more ancient, according to our present state of knowledge, than the Balanidæ. The former seem to have been at their culminant point during the Cretaceous Period, when many species of Scalpellum and Pollicipes, and a singular new genus, Loricula, existed; Pollicipes is the oldest genus, having been found in the Lower Oolite, and, perhaps, even in the Lias. The fossil species do not appear to have differed widely from existing forms. _Genus_--LEPAS. Plate I. LEPAS. _Linnæus._[22] Systema Naturæ, 1767. ANATIFA. _Brugière._[23] Encyclop. Method. (des Vers), 1789. ANATIFERA. (_Lister_) et plerumque Auctorum Anglicorum. PENTALASMIS. (_Hill._) _Leach._ Journal de Physique, July, 1817. PENTALEPAS. _De Blainville._ Dict. des Sci. Nat., 1824. DOSIMA. _J. E. Gray._ Annals of Philosophy, vol. x, 1825. [22] Linnæus, as is well known, included under this genus both the pedunculated and sessile Cirripedes. According to the rules of the British Association, the name Lepas must be retained for part of the genus; and as the sessile division was named Balanus, by Lister and Hill, even before the invention of the binomial system, and subsequently, in 1778, by Da Costa, and again, in 1789, by Brugière, there can be no question that Lepas must be applied to the pedunculated section of the genus. In this instance it is particularly desirable to recur to the Linnean name, as no other name has been _generally_ adopted. Had not Lister and Sir J. Hill published before the binomial system, their names of Anatifera and Pentalasmis would have had prior claims to Lepas. [23] The date of this publication is almost universally given as 1792, apparently caused by an error in the title-page of the First Part, which has consequently been cancelled. The First Part contains Anatifa and Balanus, and was published in 1789. The Second Part was published in 1792, and has a corrected title-page for the whole _volume_. _Valvæ 5, approximatæ: carina sursùm inter terga extensa, deorsùm aut furcâ infossâ aut disco externo terminata: scuta subtriangula, umbonibus ad angulum rostralem positis._ Valves 5, approximate: carina extending up between the terga, terminating downwards in an embedded fork, or in an external disc: scuta sub-triangular, with their umbones at the rostral angle. Filaments seated beneath the basal articulation of the first cirri; mandibles with five teeth; maxillæ step-formed; caudal appendages uniarticulate, smooth. _Distribution._--Mundane; attached to floating objects.