The Cikripedia, both recent and fossil, have been much neglected by systematic naturalists: the fossil species have, however, been more attended to than the recent. Professor Steenstrup has published an excellent monograph on the Danish and Scanian Cretaceous species: Mr. J. de Carle Sowerby has given good plates of several British valves in the Mineral Conchology; and F. Roemer has illustrated, by rather indifferent figures, though clear descriptions, various German forms. Other less important notices have appeared by several authors. As yet, however, no monograph has been produced on the whole group. The present volume is confined to the Lepadidce or Pedunculated Cirripedia; and it so happens that the introduction, under the form of notes, of a few foreign species (which are necessary to illustrate the British species), serves to render this Monograph tolerably complete; that is, as far as the specimens collected on the Continent (judging from published accounts) serve for this end,—for we shall immediately see that certain valves are requisite in each genus.

It is unfortunate how rarely all the valves of the same species have been found coembedded; it is evident that, with the exception of some few species, the membrane which held the valves together, decayed very easily, as it does in recent Pedunculated Cirripedes. Hence, in the great majority of cases, the several valves have been found separate.