[1] I feel under special obligation to Mr. Dana for several very interesting communications connected with the present subject, and for information derived from his magnificent work on the Crustacea, collected during the United States Exploring Expedition. Also to M. Bosquet, of Maestricht, for the loan and gift of several interesting fossils, described and illustrated with the utmost fidelity, in his beautiful "Monographie des Crustacés fossiles du terrain Crétacé du D. de Limbourg." MONOGRAPH ON THE CIRRIPEDIA. INTRODUCTION. My former volume, published by the Ray Society, treated only of the Lepadidæ, one family of the Cirripedia: I was induced to print it from having the materials ready, though this partial publication has been in some respects inconvenient. The Palæontographical Society has done me the honour to publish, with ample illustrations, the fossil species of this same family of Lepadidæ. This present volume completes my work on the sub-class Cirripedia.[2] I had originally intended to have published a small volume on my anatomical observations; but the full abstract given in my former volume, which will be illustrated to a certain extent in the plates appended to this volume, together with the observations here given under the Balanidæ, appear to me sufficient, and I am unwilling to spend more time on the subject. In the volume on the Lepadidæ, I gave the specific or diagnostic characters in English and Latin: I have here left out the latter, inasmuch as I have appended at the end of this volume a Latin Synopsis of all the species, recent and fossil, of the whole class. To each species is added a reference to the pages and plates of my three volumes, so that the Synopsis will serve as a systematic index to the three: an alphabetical index to the present volume is also given. In the Lepadidæ, I gave an additional specific character, derived from the softer parts of the animal's body: in the Balanidæ, these parts are more alike in the different species, and I have found it impossible to give a diagnostic character thus derived. In those cases in which a Family contains but one genus, or a Genus but one species, I have assigned my reasons for the institution of such groups, but have given, as heretofore, only a single description in full: it would have been easy to have separated, by analogy, this description into one for the species, another for the genus or for the family; but as I believe such separation and subordination of the characters would have been largely conjectural, I have thought it best to act as I have done, and give, thus saving useless repetitions, only a single description, and leave it for my successors, when more genera or species are known, to separate, with such certainty as is ever possible, the generic from the specific characters.