Oligocene System, a series of Tertiary rocks classed by Lyell as Upper Eocene, but separated by Professor Beyrich, and named from their containing a few species of Mollusca which are still living. In Britain it is perhaps solely represented by beds, formerly known as the Upper Fluviomarine series, in the north of the Isle of Wight and in the New Forest. They are thin-bedded marine; brackish, and fresh-water sands; clays, marls, and limestones, and subdivided as fellows: - Hempstead Beds. - Marls, about 160 feet thick, with a marine clay above, but mostly fresh-water and estuarine, containing Viviparus Untus, Melania, Cyrena, Unio, cypriris, and gyrogonites. Bembhidge Beds. - Estuarine marl (62 feet) above, with Ostrca vectemis, and fresh-water limestone (20 feet), with Limncea longiscata, below.