_Mouth._--The various parts vary far more than in any genus hitherto described. The labrum is highly bullate, with the upper part forming a rounded overhanging projection, and with the lower part much produced, so that the mouth is placed far from the adductor scutorum muscle, and consequently the orifice is directed more towards the ventral surface of the thorax than in most other Cirripedes: on the crest of the labrum there are some very small teeth in several of the species, but not in all. The mandibles have either three or four main teeth, generally with either one or two small teeth intermediate between the first and second large teeth, and in the case of _S. Peronii_, with small teeth between all the larger ones. The maxillæ have their edges furnished with many spines, and are either straight or have the inferior part prominent and step-formed. The outer maxillæ have the spines on their inner edges either continuous or divided into two groups, of which latter structure we have not hitherto had any very well characterised example. The olfactory orifices are either highly or moderately protuberant. In most of the species the prosoma is little developed, and the first cirrus is placed far from the second. The _Cirri_ are generally but little curled, and have elongated segments, with long, generally serrated spines: the first cirrus varies in proportional length; the second and third cirri have both their rami more thickly clothed with spines than are the three posterior cirri, the spines being generally arranged in three or four longitudinal rows: the cirri, however, of _S. villosum_ in all respects resemble closely the cirri of _Pollicipes sertus_ and _P. spinosus_. The _Caudal Appendages_ are uniarticulate, small, and clothed with spines: in _S. villosum_, however, differently from in all other allied forms, there are no appendages. The _Stomach_, in those species which I opened, is destitute of cæca. There are no filamentary appendages.