_Cirri._--Prosoma moderately developed; first pair of cirri rather far removed from the second pair. The segments of the three posterior pair are not elongated, short, slightly protuberant in front, bearing four or five pairs of strong spines; a little below each pair, there is an intermediate tuft of very fine straight bristles, of which the upper tuft is the largest; on the lateral upper rims there are some short, strong spines; dorsal tufts rather small and thick; spines all more or less serrated, especially on the broad basal segments of the three anterior cirri. Pedicels of the cirri not particularly protuberant in front. First cirrus with rami, slightly unequal in length; not short; basal segments much thicker and more protuberant than the upper segments. Second cirrus; anterior ramus with six or seven basal segments highly protuberant, and crowded with spines; posterior ramus with about six segments, similarly characterised. Third cirrus with the anterior ramus having six, and the posterior ramus five segments, also similarly characterised. _Caudal Appendages_ absent, there being only a slight swelling on each side of the anus. The _oesophagus_ runs parallel to the labrum, and enters obliquely the summit of the stomach, which is destitute of cæca: the biliary envelope is longitudinally plicated. There are no _Filamentary Appendages_. _Testes_ large, branched like a stag’s horns, attached in a sheet to the ventral surface of the stomach: the vesiculæ seminales enter the prosoma, and have their reflexed ends not very blunt. The _Penis_ is rather narrow, with the terminal half plainly ringed, and bearing tufts of fine bristles arranged in circles, one tuft below the other; on the basal half there are only a few scattered minute bristles. _Affinities._--In the downward growth of all the valves, in the presence of a sub-rostrum, in the shape of the scuta, carina, and more especially of the triangular latera, in the form of the peduncle, with its irregularly-scattered calcified scales, in the shape of the animal’s body, in the structure both of the mandibles and maxillæ, in the arrangement of the spines, both on the anterior and posterior cirri, _Scalpellum villosum_ most closely resembles, or rather is identical with, Pollicipes. Had it not been for the formation of the valves forming the capitulum, and from the presence of Complemental Males, I should have placed this species alongside of _Pollicipes spinosus_ and _sertus_. In not having caudal appendages, _S. villosum_ differs from all the species of Scalpellum and Pollicipes; but this organ is variable to an unusual degree in Pollicipes. COMPLEMENTAL MALE. Pl. VI, fig. 4.