a most conspicuous fold, which, on the view here adopted, would mark the separation of the seventh (cephalic) from the eighth (thoracic) segment; and the interspace and fold are thus simply explained. Lastly, I have shown, in the Introduction (p. 18), that the first and five succeeding pairs of cirri of the mature Cirripede present certain small, but significant, resemblances in structure and in the origin of their nerves, with the outer pair of maxillipeds and with the five pairs of ambulatory legs in the Podophthalmia; which resemblances are all futile, if the cirri belong to the 7th, 8th, 9th, 10th, 11th, and 12th segments of the cephalo-thorax, or those immediately succeeding the mouth; but are full of meaning, if the six pairs of cirri belong, as I believe, to the 9th, 10th, 11th, 12th, 13th, and 14th segments, or the six posterior segments of the cephalo-thorax. [59] 'Crustacea: United States Exploring Expedition,' p. 22. Before commencing on details, I may premise that I have examined the pupa of _Lepas australis_, _pectinata_, _fascicularis_, and _anatifera_, of _Conchoderma virgata_, partially of _Dichelaspis Warwickii_, of _Ibla quadrivalvis_, and of _Alcippe lampas_; and in the Balanidæ, of _Balanus balanoides_ and _Hameri_. In the pupæ of all these genera there is a most close general agreement in structure, excepting in minute details: I was surprised to find exactly the same slight differences in the spines on the first pair of natatory legs, as compared with the succeeding pairs, in _Balanus Hameri_, as in Lepas. The abdomen and caudal appendages of the pupa in the abnormal Alcippe, as we shall presently see, offer the only marked exception to this uniformity of character throughout the Thoracica. The outline of the carapace or shell is usually not so blunt at the anterior end, as in the pupa of _Lepas australis_ (Pl. 30, fig. 2); more commonly the shape is that of the pupa of Alcippe (Pl. 23, fig. 16). In _Lepas pectinata_ the two posterior points of the carapace are produced into two short spines. The surface of the carapace in _L. australis_ is lined, as represented in fig. 4: the colour of this species when alive was blue:[60] in _L. fascicularis_ the surface is punctured: in _L. pectinata_ it is marked with curious points of various shapes, often star-shaped, in parts reticulated, and confluent along the dorsal margin, and in parts lined: in _B. balanoides_ it is very obscurely punctured, and in _B. Hameri_ the punctures pass into lines. The whole of what is externally visible consists of the carapace, for this is produced not only backwards, so as to enclose the thorax and abdomen with their appendages, but also forwards, so as to overhang the whole front of the animal; and the prehensile antennæ, in Lepas, Ibla, Balanus, and probably in all the genera, can be retracted within its lower edge. The protection afforded by the carapace to the antennæ is aided by two crests (Pl. 30, fig. 7, _c_) parallel to this lower edge.