| Summary: | Two new species of the oregoniid crab genus Hyas are described from the Lower Pliocene of northern Japan, H. chippubetsuensis sp. nov. from the Chippubetsu Formation exposed along the Tadoshi River (Fukagawa City, central Hokkaido) and H. tentokujiensis sp. nov. from the Tentokuji Formation at Daisen City (central part of Akita Prefecture). These two new taxa both have a densely tuberculate carapace and strongly vaulted dorsal regions. Another extinct congener in the northeast Pacific region, Hyas tsuchidai Imaizumi, 1952, from the Upper Miocene Wakkanai Formation of northern Hokkaido, Japan. The earliest fossil record of the genus Hyas is from the \xe2\x80\x98lower Badenian\xe2\x80\x99 (Langhian, early Middle Miocene) of Austria. If Hyas originated in the Paratethys prior to the Middle Miocene, descendants migrated into the Pacific region by the Late Miocene and diversified during Late Miocene to Pliocene time in coolwater environments. The dispersal route of the genus might be linked to palaeobiogeographical constraints imposed by the timing of the opening of the Bering Strait.
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