_Size._--All the species are small, with a capitulum not exceeding half an inch in length. _Filamentary Appendages._--None. _Mouth._--Labrum generally considerably bullate in the upper part, with a row of teeth on the crest. The _mandibles_ have four teeth, with the inferior point narrow and spine-like, or rudimentary and absent. The _maxillæ_ have, under the two or three upper great spines, a deep notch itself bearing spines; beneath this, the lower part is straight and considerably prominent, Pl. X, fig. 15. Outer maxillæ are covered on their inner sides continuously with spines. _Cirri._--The first pair is sometimes seated very distant from the second. The arrangement of the spines on the posterior cirri varies, to an unusual degree within the limits of the same genus. We have either the ordinary structure of anterior pairs, with single fine intermediate spines (as in _P. Kæmpferi_ and _aurantia_), or we have the pairs increased by one or two additional longitudinal lateral rows, as in _P. eburnea_; or we have the front spines forming a single transverse row, as in _P. crassa_ and _P. fissa_, Pl. X, fig. 29, _a_. The segments in none of the species are protuberant; the anterior ramus of the second cirrus does not seem to be thicker than the posterior ramus, as is usually the case. The rami of the second, and of most of the other cirri, are unequal in length,--the anterior ramus, contrary to the ordinary rule, being longer in _P. eburnea_, _P. fissa_, and _P. crassa_, than the posterior ramus by several segments; I have hitherto observed this inequality only in the sessile genus Chthamalus. The _Caudal Appendages_ are small, uniarticulate, and always furnished with bristles. _Distribution._--Four out of the five species live attached to Crustacea in the European and Eastern warmer temperate and tropical oceans; the fifth species was found attached to the dead spines of an Echinus, off New Guinea. It is probable that several more species will be hereafter discovered. 1. PÆCILASMA KÆMPFERI. Pl. II, Fig. 1. _P. valvis 5; carinæ basi truncatâ et cristatâ: scutorum dentibus internis umbonalibus fortibus: tergorum acumine basali truncato, margini occludenti pæne parallelo._ Valves 5; carina with a truncated and crested base; scuta with strong internal umbonal teeth; terga with the basal point truncated, almost parallel to the occludent margin. Maxillæ with short thick spines in the notch under the two upper great spines; caudal appendages with scattered bristles on their summits, and along their whole outer margins. Japan; attached, in great numbers, to the upper and under sides of the _Inachus Kæmpferi_ of De Haan, a slow-moving brachyourous crab, probably from deep water. British Museum.