In 1862 Anderson described a parasite of Cancer pagurus from the Firth of Forth, which he named Sacculina triangularis. The species differs from Sacculina carcini in its external form and the shape of the testes, particulars of which are described in Anderson's paper in sufficient detail to prove that the parasite is specifically distinct from Sacculina carcini. In a paper dealing with the European species of the Sacculinidæ (Boschma, 1927) I mentioned Sacculina triangularis as a distinct species, basing this statement on Anderson's description and figures. Afterwards I had the opportunity to study the anatomy of the species, owing to the kindness of Professor J. H. Ashworth, who sent me nine specimens of Sacculina on Cancer pagurus from the Firth of Forth. The following description of the characters of the species is based on this material.