_Mouth with the labrum notched in the middle, not swollen; palpi large, almost touching each other; mandibles generally with the lower teeth laterally double; third pair of cirri with their segments resembling those of the second pair._ _First Section._† _Scutum and tergum articulated together, or overlapping each other; each branchia composed of a single plicated fold._ _Genera_--Balanus; Acasta; Elminius; Tetraclita; Pyrgoma; Creusia; Chelonobia. _Second Section._†† _Scutum and tergum (when both are present) not overlapping each other; basis membranous; parietes often deeply folded, with the outer lamina, towards the basis, generally imperfect; each branchia composed of two plicated folds; shell attached to living vertebrata._ _Genera_--Coronula; Platylepas; Tubicinella; Xenobalanus.[82] [82] At the end of the volume a Synopsis is given, which will serve as a systematic index for the discovery of generic and specific names. The Balanidæ may be divided into two sub-families; namely, the Balaninæ and Chthamalinæ; and, in the former, the genera, as we see, may be very naturally grouped into two sections. The Balaninæ differ from the Chthamalinæ, as far as the shell is concerned, in the rostrum having radii but no alæ, all the lateral compartments having both radii and alæ; on the other hand, in the Chthamalinæ, the rostrum has alæ, and the rostro-lateral compartments radii on both sides, and therefore no alæ. These differences probably arise, as already explained, from the perfect confluence, in the Balaninæ, of the true rostrum with the rostro-lateral compartments. In Chelonobia, belonging to the Balaninæ, we see an intermediate state, with the fusion not quite effected: on the other hand, in one genus amongst the Chthamalinæ, namely, Pachylasma, we must look to the shell at a very early age, to find the rostrum with its alæ, distinct from the rostro-lateral compartments. In Tetraclita, Elminius, and Creusia, the carino-lateral compartments are aborted, or possibly confluent with the lateral compartments, making altogether only four: in Pyrgoma all the compartments are fused together and form a solid ring. The sub-genus Acasta is, in one sense, very natural, as it includes species most closely allied: in another sense it is far from natural, as some of the species can hardly be distinguished from those species of Balanus, which live attached to Gorgoniæ: I almost regret I did not merge the species of Acasta into Balanus. In the Balaninæ generally the parietes are either porose, or are furnished on their internal surfaces with regular ribs, representing the longitudinal parietal septa, which in other species form the tubes or pores; there are, however, many exceptions to this rule in several species of Balanus, in Acasta and Elminius, all of which have the parietes of their shells internally quite smooth, or only irregularly roughened with points.