Abstract
A new Rimoneis species, R. ectorii Marquardt & C.E.Wetzel sp. nov., was observed during a palaeoecological study of a sediment core from Pleistocene sediments of the Colônia basin (southeast Brazil). Rimoneis ectorii is formally described based on light and scanning electron microscope observations. The new species may be distinguished from the two other species ascribed to date to the genus by: (i) its rhombic-lanceolate valve outline, (ii) the presence of robust, pointed, and irregularly distributed spines on the valve face margin, (iii) a higher areola density towards the valve ends, and (iv) a greater valve width. Our findings expand the knowledge about the genus Rimoneis and its geographic distribution. Additional investigations are necessary to identify other sites in which this genus may be present, since recent records show unknown and/or previously neglected species.