Mrs. EVELINE DU BOIS-REYMOND MARCUS found the turbellarian described below, which belongs to the Umagillidae (Neorhabdocoela, Dalyellioida), in a vial of Dr. P. WAGENAAR HUMMELINCK’S collection of Caribbean polyclads. The species of Syndesmis are known to live in the body cavity and the intestine, especially the rectum, of sea-urchins, so that the present specimens might have been evacuated together with the faeces. The last revision of the Umagillidae (STUNKARD & CORLISS 1951) was followed by papers of WESTBLAD (1953), HICKMAN (1955, 1956), HICKMAN & OLSEN (1955), and HYMAN (1960). Their results suggest continuing the key of STUNKARD & CORLISS. AS the present species has a simple, not H-shaped, intestine, paired testes, and paired ovaries, the supplement to this KEY is restricted to STUNKARD & CORLISS’ numbers 1-15 of the subfamily Umagillinae.