During my excavations in the marshes of Sirabé (Vakinankaratra District, Central Madagascar), my assistant, Mr. A. Robert, came upon a small fragment of a right upper jaw of a mammal, containing the two anterior true molars, which presented the well-known pattern of the Old-World Cercopithecidæ. The presence of true monkeys, as contemporaries of the Æpyornis, in very recent deposits, appeared to be such an extraordinary fact that I anxiously looked forward for further and more conclusive evidence. This fortunately has since been forthcoming, by the subsequent discovery of more complete remains. Two of the most important pieces, viz. the anterior part of a skull, broken off behind the nasals and the molar series, and a left mandibular ramus, are here figured and briefly described.