When writing my introduction to Dr. Jentink’s treatise on the Mammals collected during my sojourn in Borneo (N. L. M. XIX (1897), pp. 1—26), I had no idea that the publication of the present paper, which at that time was nearly ready for the press, would have to undergo such a long delay. But the duties of my new position as Director of the Zoological Garden at Rotterdam absorbed so much of my time that I could not think much about finishing my manuscript. And even now I am, for want of time, unable to do what I originally had proposed myself to do, i. e. to give a critical review of the birds of Borneo, but must content myself with an enumeration of the species obtained by myself and my companions, especially by my assistant Mr. Max Moret, who, after my return to Europe, made a small collection of birds in the surroundings of Pontianak near the west coast, and of Dr. A. W. Nieuwenhuis 1), who, from May 1896 to May 1897 and from November 1898 to March 1899, made collections of birds on the Upper Kapoeas 1) and on the River Mahakkam (East-Borneo) and its tributaries. Moreover I am indebted to Mr. L. Westenenk, an officer in the Dutch colonial civil service, at that time »Assistent Resident” at Sanggau, for a small collection of birds preserved in spirits, from the junction of the Sekajam with the Kapoeas River and sent to me after my return to Leyden.