PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON 110(3):476-482. 1997. A new polyclad flatworm, Tytthosoceros inca (Plathyhelminthes: Polycladida: Cotylea: Pseudocerotidae), from Chilean coastal waters. Juan A. Baeza, David Veliz, Luis M. Pardo, Karin Lohrmann, and Chita Guisado Universidad Catolica del Norte, Sede Coquimbo, Facultad de Ciencias del Mar, Departamento de Biologia Marina, Casilla 117, Coquimbo, Chile Abstract. — A new polyclad species, Tytthosoceros inca is described from Chilean coastal waters. It resembles Pseudoceros luteus (Plehn, 1898) Hyman 1953 incerta sedis but differs in having ear-like pseudotentacles, a brown mar-ginal band, and dark brown spots on the dorsal surface of the body. Tyttho-soceros inca is associated with the colonial ascidians Ciona intestinalis and Pyura chilensis, living on or near them. This is the first record of the genus from Chilean coastal waters and the second report of the family from cold temperate waters, although this time from the southern hemisphere. Pseudocerotidae is one of the most pop-ulous families of polyclad flatworms (Hy-man 1954, Newman & Cannon 1994). Mar-cus (1950) cited an extensive list of over 70 species, mainly Pseudoceros sensu lato, de-scribed from around the world. Hyman (1959a) extended the list to approximately 120, adding many species overlooked by Marcus (1950) and others described by her. Another 60 new species described by Hy-man (1959b), Faubel (1984), Prudhoe (1981, 1989), and Newman & Cannon (1994, 1995, 1996a, 1996b) must now be added to the list of species described world-wide, increasing the total number of known species to at least 200. Most species of the family occur in trop-ical and subtropical waters of the Indo-Pa-cific region (Hyman 1954). They have been reported less frecuently from warmer tem-perate waters around the world, and only one species, Pseudoceros canadensis Hy-man, 1953 from cold temperate waters around British Columbia, Canada (Hyman 1953). However, this species is considered incerta sedis due to its incomplete descrip-tion (Faubel, 1984; Newman & Cannon, 1984). At present, the following 14 pseudocer-otid genera are recognized; Thyzanozoon Grube, 1840, Acanthozoon Collingwood, 1876, Pseudoceros Lang, 1884, Yungia Lang, 1884, Nymphozoon Hyman, 1959, Cryptoceros Faubel, 1984, Cryptobiceros Faubel, 1984, Monobiceros Faubel, 1984, Pseudobiceros Faubel, 1984, Parapseudo-ceros Prudhoe, 1989, Bulaceros Newman & Cannon 1996, Tytthosoceros Newman & Cannon 1996, Maiazoon Newman & Can-non 1996, and Phrikoceros Newman & Cannon 1996. They can be easily disti-guished by details of the male and female reproductive systems (presence or absence of prostate and seminal vesicle, and the number of gonopores and complexes), the alimentary system (presence or absence of anal pores), and by external characters, such as the shape of the body, pseudoten-tacles, pharynx and pharyngeal lobes, and the arrangement of the cerebral and pseu-dotentacular eyes (Newman & Cannon 1994, 1995, 1996a, 1996b). Nevertheless, species recognition and differentiation within each genus presents problems in this group, like in many others that lack solid structures. Marcus (1950),
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