The species is characterized by 1) cap-shaped, moderately high shell attaining 13 mm in length,
2)apex coiled to the right, slightly projecting beyond the posterior end, 3) protoconch small, slightly heterostrophic, of about 1 whorl, 4) aperture large, broadly oval, lying on a concave surface and with a sloping inward columellar shelf, 5) outer surface with coarse growth markings and uneven spiral ribs, 20 or more in number.
DISTRIBUTION AND HABITAT. Amathina oyamai oyamai is so far known only as a fossil. The type-material of the Recent subspecies mortoni Ponder, 1987, was recovered in Hong Kong waters, at about 4 m depth, living on Crassostrea shells (PONDER, 1987). The present subspecies is supposed to have parasitized turriculate gastropods on account of the concave base (PONDER, 1987).
FOSSIL RECORDS. Quaternary of Japan; Holocene of Thailand.