971. PLAGIACANTHIDAE (RADIOLARIA) FROM THE UPPER MIOCENE OF EASTERN HOKKAIDO, JAPAN

Abstract

Well-preserved radiolarians are abundant in diatomaceous siltstones of the Taiki Formation (Upper Miocene) in the Toyokoro Hills region, and the Chokubetsu, Atsunai and Shiranuka Formations (Middle Miocene to Lower Pliocene) in the Atsunai Coast region, eastern Hokkaido, Japan. Fifteen samples are studied from five routes ; Oikamanai River, Yudou River, Noyaushi River, Ishii-zawa River and Atsunai Coast routes. Plagiacanthidae species are a principle component of these assemblages. Seven genera of Lophophaeninae (including two new genera and six new species), two of Clathromitrinae and one of Sethoperinae are described. Three groups of Plagiacanthidae having characteristic internal skeletal structures are recognized.·Group 1 : The first segment is distinguished from the second segment by only AL.·Group 2 : The first segment is distinguished from the second segment by AL and VL (instead of AL and LL when V is lacking).·Group 3 : The first segment is distinguished from the second segment by DL and VL, or AL-AL, AL and VL, or DL and LL. All genera of groups 1 and 2 are included in Lophophaeninae, while two genera of group 3 are in Clathromitrinae, one is in Sethoperinae and one is in Lophophaeninae.