A further detailed description of the animal is here given. The Synaptid Protankyra bidentata, whose length may attain 30 cm, lives in a tube lying nearly horizontally about 8-10 cm below the surface of the muddy bed. Usually only one Entovalva is found attached to the host, and from among 134 Synaptids collected and examined, 53 of the commensals (40 per cent.) were found. Montacuta, Hexapus, and three species of Polychaetous Annelids are also often found living with this Synaptid as commensals. Hakosaki near Fukuoka, of Hakata Bay of northern Kyushu is added, besides Tomioka of Amakusa, as another habitat of this bivalve. Entovalva semperi in situ does not move actively, but if removed from the host it creeps about, often with a regular rhythm of contraction and extension of the foot and body. The whole surface of the creeping sole seems to act as sucker, but hardly any scar is left on the skin of the Synaptid if forcibly detached from it. Internal anatomy of each organ as studied from sections is described in detail, and is compared with the accounts of Entovalva mirabilis and perrieri given by VOELTZKOW, MALARD, ANTHONY, NIERSTRASZ, etc. No individual with testis has yet been examined, so that it is not decided if the animal really is hermaphroditic. MALARD'S suggestion that the food may be taken in through the byssal papilla, and ANTHONY'S idea of incipient division of the siphonal orifice, are both disproved. The presence of teeth in the valves of mirabilis and Devonia donacina makes the interrelations of these allied forms rather intricate, but for the present the author prefers to retain the genus Entovalva to comprise the three species, mirabilis, perrieri and semperi.