Two new marine gastrotrichs of the genus Ptychostomella (Macrodasyida, Thaumastodermatidae) from South Korea.

Two new thaumastodermatid gastrotrichs belonging to the genus Ptychostomella are described from the shallow sublittoral sand bottom in the eastern coast of South Korea. In having the embossed cuticular armature, Ptychostomella orientalis n. sp. is most allied to P. lepidota Clausen, 2000, the only congeneric species so far known as possessing such a cuticular armature. Ptychostomella papillata n. sp. is characterized by the terrace-shaped cuticular protrusion on head region, and the numerous papillae with sensory hair(s). Description of the two new species is prepared with a character comparison table and a key to the species of Ptychostomella. This is the first record of genus Ptychostomella from the North Pacific.