FIG. 6 in Two new species of Achnanthidium Kützing (Achnanthidiaceae) from the Quaternary sediments of the Colônia basin, Southeast Brazil

FIG. 6. — Achnanthidium craterianum Marquardt & C.E.Wetzel, sp. nov. Population from the type locality. SEM: A-D, external views, raphe valve; C, detail of terminal raphe fissure and areolae; D, detail of central raphe fissures and central area; E, F, external views, rapheless valve. Scale bars: A, C, D, 1 µm; B, E, F, 3 µm.