Aulacoseira praegranulata var. praeislandica (Jousé) Moisseeva ofthe early Miocene Masaragawa Formation, Sado Island, Niigata, Japan

Abstract

The diatoms of the early Miocene Masaragawa Formation distributed around Sado Island, Niigata, Japan were investigated. The formation contains diatomaceous mudstone yielding an abundance of Aulacoseira taxa, such as Aulacoseira praegranulata var. praeislandica (Jousé) Moisseeva and A. praegranulata var. praeangustissima (Jousé) Moisseeva. Aulacoseira praegranulata var. praeislandica has three forms of coarsely areolated, densely areolated and thickly formed valves joined in the same chain. Three forms of Aulacoseira praegranulata var. praeislandica and accompanying taxa are shown with LM and SEM.