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A copious Silurian biota was described by Sugiyama (1940) from the Kawauchi formation at Sakari-machi, Of unato City, Iwate Prefecture. He distinguished there five horizons, namely Solenopora limestone, Encrinurus bed, Halysites limestone, Clathrodictyon limestone and Favosites limestone in descending order and determined its Middle Silurian age. Through a study on the Halysitidae in Japan, however, the junior author (1958) reached the conclusion that the Schedohalysites kitakamiensis (Sugiyama) in the Clathrodictyon and Halysites limestones indicates a lower Ludlovian age. Therefore the authors (1974) dated the Encrinurus bed overlying them at middle Ludlovian. On the contrary to the profusion of corals and stromatoporoids Encrinurus kitakamiensis Sugiyama, 1941, was a sole trilobite. Therefore Sphaerexochus (Onukia) sugiyamai here described is the second Silurian trilobite in North Japan. It is of great interest not only by rarity, but also by high specialization in complete effacement of two anterior furrows on the glabella, fusion of the axial ring with the terminal piece in the pygidium, absent genal and pleural spines and large size of the shield.

A new Silurian Trilobite from Ofunato, North Japan