By his decease the Leyden Museum of Natural History has lost a Director, who during nearly thirty years has given all his energy to this Institution, has tried to extend and to complete its collections and to bring its internal arrangement more in agreement with modern views. In the first years of his employment at the Museum (Conservator, 1875—84) he had the good fortune to study under the guidance of Hermann Schlegel and to be introduced by him in the System of the Mammals; for to this class he applied nearly all his scientific labour and he has published a great number of papers on them. Especially by his publications on the Bats and the Rodentia he became known as a learned and accurate scientific investigator. However a good deal of his time in the latest years of his life has been devoted to the foundation of a new Museum, in which he endeavoured to realise the idea of a complete separation between the scientific and the public collections and thus to effectuate „das Ideal eines naturhistorisehen Museums”. His opinion regarding this question he explained in a communication to the International Congress of Zoology at Bern; for he was not only an assiduous visitor of these international assemblies, but as a President of the Congress at Leyden in 1895, he was also a member of the Comité Permanent. Returning from the latest congress at Monaco he showed the first traces of an illness, that in a few months has ruined his vigorous constitution.