The genus may be divided into two sections, firstly, _P. Kæmpferi_ and _P. aurantia_, which have their carinæ basally truncated, the basal angles of their terga cut off, and the anterior rami of their second cirri shorter than the posterior rami; and, secondly, _P. crassa_, _P. fissa_, and _P. eburnea_, which in these several respects are otherwise characterised. The _P. eburnea_, however, differs rather more from _P. crassa_ and _P. fissa_, than these two do from each other; but certainly not enough to allow of the retention of Mr. Hinds’ genus of Trilasmis. _P. crassa_, in an especial degree, connects together all the forms. _General Appearance._--Capitulum oval, more or less produced, flat or gibbous; formed of three, five, or seven approximate valves; the lesser number arising from the abortion of the terga, and the greater number from the scuta being divided into two segments. Valves moderately thick, either white or reddish, smooth or striated, and sometimes partly covered by membrane, bearing minute spines. _Scuta_ oval, of varying proportions; the basal margin is generally narrow, and blends into the carina-tergal margin; the internal basal rim generally is well developed, sometimes with, and sometimes without internal teeth beneath the umbones. In _P. eburnea_, and sometimes in _P. crassa_, there is a line of apparent fissure, and in _P. fissa_ of actual disseverment, running from the umbo to the apex of each scutum, nearly in the line in which a ridge extends in Lepas: the primordial valves of the scuta in these three species, are seated at the basal angles of the lateral and larger segments. The positions of the primordial valves, and the direction of growth in the calcified valves, are, in all the species, the same as in Lepas. In several of the species attached to Crustacea, the two scuta are unequally convex, which is caused, as was pointed out to me by Mr. Gray, by that valve which lies close and nearly parallel to the body of the crab, being least developed. The _Terga_ are either quite absent, or rudimentary as in _P. crassa_, or pretty well developed as in the other species: the occludent margin is single, and not double as generally in Lepas; the basal angle is either pointed or truncated. The _Carina_ varies considerably in shape, but never extends up between the terga, nor ends downwards in a fork; in the first two species it is truncated; in the others, it terminates in a deeply-imbedded oblong disc, which in _P. eburnea_ seems almost entirely (but of course not quite) to separate the inside of the capitulum from the peduncle; a similar separation is effected in _P. fissa_, where the imbedded disc is small, by two large teeth on the internal basal rims of the two scuta. The carina is always narrow, and either solid internally or very slightly concave. _Peduncle_, is very short and narrow; the membrane is generally ringed with thicker, yellower portions, and often bears very minute spines.