The present paper contains the chief particulars of a specimen in the collection of the United States National Museum, differing from the hitherto described species of Sacculina in its anatomical characters and in those of the cuticle of the mantle. Sacculina punctata nov. spec. Kaseda, Satuma, Japan, T. Urita coll. et don., 1 specimen on Plagusia dentipes de Haan, U. S. N. M. N°. 48337. Diagnosis. Testes almost entirely in the posterior part of the body, outside the visceral mass, vasa deferentia partially in the visceral mass. The two testes well developed, completely separated, forming more or less globular or somewhat compressed, wide sacs, which are connected with the wide vasa deferentia by a short, narrow tube. Colleteric glands with numerous branched tubes, approximately in the central part of the lateral surfaces of the visceral mass. External cuticle of the mantle without excrescences, or covered with small roundish papillae, or provided with irregular outgrowths which give it a more or less ragged appearance. Surface of the external cuticle with distinct little areas which have a diameter varying from 7 to 18 µ. Usually in these areas there is a central column consisting of chitin which differs in structure from that of its surroundings. Internal cuticle with numerous retinacula, arranged in rows on its surface. Each retinaculum consists of a single spindle which has a length of 7 to 12 µ. The specimen is comparatively large, its breadth (greater diameter) is 23 mm, its height (the distance from the stalk to the mantle opening) is 16 mm, and its thickness 6 mm. The animal is more or less kidney-