Abstract
The taxonomy on two Recent species of the genus Orectospira and their bathymetrical distributions were studied on a precise data of depths and locations off the coast of Japan. Orectospira Dall, 1925, a small gastropod genus of family Orectospiridae, is known to have a subtle difference between adult and juvenile shells. Through the examination of shell-height/shell-width ratio and characteristics of growth lirae, unbilicus and siphonal canal among three species described so far, it became clear that only O. tectiformis (Watson, 1880) and O. shikoensis (Yokoyama, 1928) were valid Recent Japanese species belonging to the genus Orectospira and that O. babelica (Dall, 1907) is a synonym of O. tectiformis. This synonymization was also supported by bathymetric distributions of above species, as both O. tectiformis and O. babelica were found in the same depths from 700 to 1100 m. On the other hand, O. shikoensis was collected from depths between 150 and 800 m, especially around 300 to 600 m, and consequently a clear boundary of the bathymetrical distribution between O. tectiformis and O. shikoensis exists at depths between 700 and 800 m.
