Page. Introduction 1 Descriptions of skulls and skeletons of Ziphioid whales 3 Genus Mesoplodon 3 Mesoplodon bidens 4 densirostris 9 europæus 11 stejnegeri 24 Genus Ziphius 30 Ziphius cavirostris 30 Genus Berardius 60 Berardius bairdii 60 Genus Hyperoödon 76 Hyperoödon ampullatus 76 List of species of existing Ziphioid whales 76 Index 79 Explanation of plates 83 AN ACCOUNT OF THE BEAKED WHALES OF THE FAMILY ZIPHIIDÆ IN THE COLLECTION OF THE UNITED STATES NATIONAL MUSEUM, WITH REMARKS ON SOME SPECIMENS IN OTHER AMERICAN MUSEUMS. By Frederick W. True, _Head Curator, Department of Biology, U. S. National Museum._ INTRODUCTION. The beaked whales belonging to the family Ziphiidæ are, with the exception of the bottle-nosed whales of the genus _Hyperoödon_, among the rarest of cetaceans. Of the three genera _Mesoplodon_, _Ziphius_, and _Berardius_, so far as I have been able to ascertain from published records, specimens representing about one hundred individuals are known, and somewhat more than one-half of these belong to the first-named genus. _Berardius_ is the rarest genus, only about fourteen specimens having been collected thus far. The U. S. National Museum contains specimens representing some twenty-five individuals of the three genera, or about one-fourth of the material at present available. Among these are six specimens of the genus _Berardius_, or nearly half of all that have been recorded thus far.