Diagnosis. Aplysiidae of conical form, narrower in front, wide and obliquely truncate behind. Integument more or less warty, the warts bearing villi; warts and villi being wholly retractile. Parapodia united, save for a dorsal slit; the free lobes covering the slit leave two conspicuous respiratory openings, one at the posterior and one at the anterior 'end. The posterior respiratory opening, corresponding to the mantle-siphon, lies about in the middle of the posterior disc, the anterior opening outside the disc, somewhat in front of the disc's anterior margin. Shell solid, hatchetshaped, the free spire calloused. Penis armed or unarmed. Radula with inconspicuous, often reduced rhachidian tooth and many laterals with long simple cusps. Short history of the genus Though most authors consider it, with Cuvier (1804) a discovery of Peron, that Rumphius' animal of plate X belongs to the shell of his plate XL, this seems not true. If we compare the text on p. 38 on Limax (marina) tertia with that on p. 122-3 on the Opercula callorum, it is clear that Rumphius was speaking of the same species of mollusc, that the only mistake Schijnvoet, the editor of Rumph's M.S., made, was that he did not refer to p. 38 when Rumphius on p. 122 remarked: "zijnde een slach van den Limax marina". The "third operculum" lies in the flesh and looks like a ham ("schonkje van een varken", which may be a scapula, as some have interpreted it, or a ham, as Rumphius himself said on p. 38). All details on the taste and smell and on the habitat are the same on p. 38 and p. 123. We may conclude that Rumphius intended the same animal, though the description of the animal and that of the shell were each