Birds (Aves) from the Middle Miocene of Arrisdrift (Namibia). Preliminary study with description of two new genera: Amanuensis (Accipitriformes, Sagittariidae) and Namibiavis (Gruiformes, Idiornithidae). Geological Survey of Namibia Memoir, 19 2003: 103-113. 106 [Zoological Record Volume 140]