Whoever will refer to the published descriptions of recent and fossil Cirripedia, will find the utmost confusion in the existing nomenclature: thus, the valve named in the woodcut the _Scutum_, has been designated by various well-known naturalists as the “ventral,” the “anterior,” the “inferior,” the “ante-lateral,” and the “latero-inferior” valve; the first two of these titles have, moreover, been applied to the rostrum or rostral valve of sessile Cirripedes. The _Tergum_ has been called the “dorsal,” the “posterior,” the “superior,” the “central,” the “terminal,” the “postero-lateral,” and the “latero-superior” valve. The _Carina_ has received the first two of these identical epithets, viz. the “dorsal” and the “posterior;” and likewise has been called the “keel-valve.” The confusion, however, becomes far worse, when any individual valve is described, for the very same margin which is anterior or inferior in the eyes of one author, is the posterior or superior in those of another; it has often happened to me that I have been quite unable even to conjecture to which margin or part of a valve an author was referring. Moreover, the length of these double titles is inconvenient. Hence, as I have to describe all the recent and fossil species, I trust I may be thought justified in giving short names to each of the more important valves, these being common to the pedunculated and sessile Cirripedes. The part supported by the peduncle, and which is generally, though not always, protected by valves, I have designated the _Capitulum_. The title of _Peduncle_, which is either naked or squamiferous, requires no explanation; the scales on it, and the lower valves of the capitulum, are arranged in whorls, which, in the Latin specific descriptions, I have called by the botanical term of verticillus. I have applied the term _Scutum_ to the most important and persistent of the valves, and which can generally be recognised by the hollow giving attachment to the adductor scutorum muscle, from the resemblance which the two valves taken together bear to a shield, and from their office of protecting the front side of the body. From the protection afforded by the two _Terga_ to the dorso-lateral surface of the animal, these valves have been thus called. The term _Carina_[2] is a mere translation of the name already used by some authors, of Keel-Valve. [2] In the Carina of Fossil Species of Scalpellum, I have found it necessary to distinguish different parts, viz., A, the tectum, of which half is seen; B, the parietes; and C, the intra-parietes. The _Rostrum_ has been so called from its relative position to the carina or keel. There is often a _Sub-carina_ and a _Sub-rostrum_.