In Monographs, it is the usual and excellent custom to give a history of the subject, but this has been so fully done by Burmeister, in his ‘Beiträge zur Naturgeschichte der Rankenfüsser,’ and by M. G. Martin St. Ange, in his ‘Mémoire sur l’Organisation des Cirripèdes,’ that it would be superfluous here to repeat the same list of authors. I will only add, that since the date, 1834, of the above works, the only important papers with which I am acquainted, are, 1st. Dr. Coldstream ‘On the Structure of the Shell in Sessile Cirripedes,’ in the ‘Encyclopædia of Anatomy and Physiology;’ 2d. Dr. Lovén ‘On the Alepas squalicola,’ (‘Ofversigt of Kongl. Vetens.,’ &c. Stockholm, 1844, p. 192,) giving a short but excellent account of this abnormal Cirripede; 3d. Professor Leidy’s very interesting discovery, (‘Proceedings of the Academy of Natural Sciences,’ Philadelphia, vol. iv, No. I, Jan. 1848,) of eyes in a mature Balanus; 4th. Mr. A. Hancock’s Memoir, (‘Annals of Natural History, 2d series, Nov. 1849,) on his Alcippe lampas, the type of a new order of Cirripedes; 5th. Mr. Goodsir’s Paper, (‘Edinburgh New Philosoph. Journal,’ July 1843,) on the Larvæ in the First Stage of Development in Balanus; 6th. Mr. C. Spence Bate’s valuable Paper on the same subject, lately published, (Oct. 1851,) in the ‘Annals of Natural History;’ and lastly, M. Reinhardt has described, in the ‘Copenhagen Journal of Natural History, Jan. 1851,’ the _Lithotrya Nicobarica_, and has discussed its powers of burrowing into rocks. I have given the specific or diagnostic characters, deduced from the external parts alone, in both Latin and English. As I found, during the progress of this work, that a similarly abbreviated character of the softer internal parts, was very useful in discriminating the species, I have inserted it after the ordinary specific character. In those cases in which a genus includes only a single species, I have followed the practice of some botanists, and given only the generic character, believing it to be impossible, before a second species is discovered, to know which characters will prove of specific, in contradistinction to generic, value.