Abstract
Fine structure of Aulacoseira valida (GRUNOW in VAN HEURCK) KRAMMER was examined using light and scanning electron microscopy. Plenty of frustules of this species were found in the peat and clay layers inserted between the Shimosueyoshi Formation and the Shimosueyoshi Loam Formation of the Upper Pleistocene assumed to be 0.12 to 0.13 million years old.
Though the thick valve mantle and big spines on the face/mantle juncture were the only noticeable characteristics of this species in light microscopical observations, scanning electron microscopy revealed the following features.
1) T-shaped spine composed of a long and hollow stalk, an extended solid distal end and a solid base arising from every interareolar broad costa on the face/mantle juncture. 2) mantle wall composed of outer and inner layers. The outer layer contains areolae and the underlying inner layer consists of entangled siliceous threads forming a sponge-like structure. 3) well developed thick and solid ringleiste. 4) usually one but sometime sup to three labiate processes on the inner surface of the ringleiste facing the valve face. The labiate process opens to the outside at the proximal end of a mantle areolar row. 5) cingulum composed of five open bands.
