Two new ostracod species, Semicytherura maxima n. sp. and S. ikeyai n. sp., both belonging to the S. henryhowel group of the genus, are described. They were collected from Akkeshi Bay in northeastern Japan, and inhabit the marine sediment surface in places deeper than the intertidal zone. Their distributions in northern Japan seem to be influenced by the cold-water Chishima Current (Oyashio). The geological distribution and species diversity were surveyed for each of the subgroups recognized in the S. henryhowei group. The results suggest that these subgroups split from each other in the NW Pacific by the Early Miocene, and that one of them has spread around the Northern Hemisphere, while the other has remained in the NW Pacific since that time.