Z Ziphiidæ from east coast of United States, 2 from west coast of United States, 2 list of existing species of, 76 specimens of, available for study, 1 in National Museum, 1 _Ziphius_, 30, 77 fossil, 4 species of, 30 _Ziphius cavirostris_, 2, 30 Argentine specimen of, 36 caudal vertebræ of, 42 cervical vertebræ of, 38 chevron bones of, 44 color of, 33, 34 comparison of skeletons of, 36 dimensions of, 32 distribution of, 77 earbones of, 83 external characters of, 59 external dimension of, 32, 33, 34 from Argentina, 55, 57 from Barnegat City, N. J., 2, 31 from Barnegat City, N. J., history of, 33 from Bering Island, 1, 31 from Charleston, S. C., 2, 31 from Kiska harbor, Alaska, 1, 2, 31 from Newport, R. I., 2, 31, 32 from St. Simon Island, Ga., 2, 31 lumbar vertebræ of, 41 pectoral limb of, 46 phalangeal formula of, 46, 49 scapula of, 45 sex characters of, 54 skeleton, dimensions of, 47 skeleton of, from Bering Island, 58 skull, age variations in, 50 dimensions of, 53 sternum of, 45 teeth, description of, 55 dimensions of, 55 thoracic vertebræ of, 40 skeleton, variations in, 49 vertebræ of, 37 vertebral column of, 36 vertebral formula of, 36 _Ziphius gervaisii_, 30, 54 _Ziphius grebnitzkii_, 30 external characters of, 59 from Bering Island, 31 skull, dimensions of, 53 skeleton of, from Bering Island, 58 _Ziphius semijunctus_, 30, 35 from Charleston, S. C., 2 type-skull, dimensions of, 53 _Ziphius seychellensis_, 9 EXPLANATION OF PLATES. [Illustration: Plate 1 SKULLS OF MESOPLODON] Fig. 1. _Mesoplodon bidens._ Skull. Nantucket, Mass. Mus. Comp. Zoölogy, No. 1727. Female, adult. Dorsal aspect. About ¼ nat. size. Extremity of beak defective. 2. _Mesoplodon densirostris_? Skull. Annisquam, Mass. Female, young. Boston Society of Natural History. Dorsal aspect. ¼ nat. size. Defective on the left side. [Illustration: Plate 2 SKULLS AND TOOTH OF MESOPLODON] Fig. 1. _Mesoplodon europæus._ Skull. Atlantic City, New Jersey. Male, young. Cat. No. 23346, U.S.N.M. Dorsal aspect. ¼ nat. size. 2. _Mesoplodon europæus._ Skull. North Long Branch, New Jersey. Female, adult. Mus. Comp. Zoölogy. Dorsal aspect. ¼ nat. size. Distal portion of beak lacking and right frontal region defective. 3. _Mesoplodon bidens._ Tooth. Nantucket, Mass. Mus. Comp. Zoöl., No. 1727. Nat. size. [Illustration: Plate 3 SKULLS OF MESOPLODON STEJNEGERI] Fig. 1. _Mesoplodon stejnegeri._ Type-skull. Bering Island. Immature. Cat. No. 21112, U.S.N.M. Dorsal aspect. ¼ nat. size. Edges abraded; distal end of beak defective.