During a visit to the Rijksmuseum van Natuurlijke Historie in Leiden in July 1964, Dr D. A. Hooijer kindly allowed me to re-examine the mandibular fragment which had been discovered by Eugène Dubois on 24 November 1890 at Kedung Brubus, a fossil locality in the Kendeng Beds in Central Java (Dubois, 1891a, 1891b, 1924a, 1924b). As a result of a possibly important new point which emerged during my examination of the specimen, Dr Hooijer urged me to place my observations and their implications on record. It is the purpose of this short paper to do so. BRIEF HISTORICAL SYNOPSIS In his original two notes of 1891, Dubois clearly recognized the fragment as belonging to "Homo spec. indet." He drew attention to the poor chin development and the curious flattening and hollowing of the inner surface, which he attributed to the attachment of the Musculus digastricus. On this basis, he spoke of the fragment as representing "...eene andere en waarschijnlijk lagere type dan eenige die men kent" (1891a). Thirty-three years later, Dubois described (1924a) and beautifully illustrated (1924b) the little jaw fragment. At this time he definitely associated it with the Trinil hominid fossils and regarded it as part of the hypodigm of Pithecanthropus erectus. A passing reference was made to this "very peculiar human mandible" in the course of his description of the Giant Pangolin of the Kendeng fauna (Dubois, 1926), while the description and illustrations were repeated in Dubois (1938). The mandible (or a cast) was studied by McGregor (1925), Weinert (1928) and Weidenreich (1936). McGregor accepted it as a jaw of P. erectus; Weinert reserved judgment until definite Pithecanthropus mandibles should be discovered. Weidenreich,