2. _Mesoplodon stejnegeri._ Skull. Yaquina Bay, Oregon. Adult. Cat. No. 143132, U.S.N.M. Dorsal aspect. ¼ nat. size. Proximal end of premaxillæ defective and right nasal lacking. [Illustration: Plate 4 SKULLS OF MESOPLODON] Fig. 1. _Mesoplodon bidens._ Skull. Nantucket, Mass. Female, adult. Mus. Comp. Zoöl. No. 1727. Ventral aspect. About ¼ nat. size. Tip of beak, left pterygoid, and malars defective. 2. _Mesoplodon densirostris_? Skull. Annisquam, Mass. Female, young. Boston Society of Natural History. Ventral aspect. ¼ nat. size. Left frontal region defective. [Illustration: Plate 5 SKULLS OF MESOPLODON EUROPÆUS] Fig. 1. _Mesoplodon europæus._ Skull. Atlantic City, New Jersey. Male, young. Cat. No. 23346, U.S.N.M. Ventral aspect. ¼ nat. size. 2. _Mesoplodon europæus._ Skull. North Long Branch, New Jersey. Female, adult. Mus. Comp. Zoölogy. Ventral aspect. ¼ nat. size. Distal portion of beak lacking, pterygoids, malars, and left frontal and temporal regions defective. [Illustration: Plate 6 SKULLS OF MESOPLODON STEJNEGERI] Fig. 1. _Mesoplodon stejnegeri._ Type-skull. Immature. Cat. No. 21112, U.S.N.M. Ventral aspect. ¼ nat. size. Edges abraded; tip of beak, pterygoids, zygomatic processes, etc., defective. 2. _Mesoplodon stejnegeri._ Skull. Adult. Cat. No. 143132, U.S.N.M. Ventral aspect. About ¼ nat. size. Pterygoids and left malar defective. [Illustration: Plate 7 SKULLS OF MESOPLODON] Fig. 1. _Mesoplodon bidens._ Skull. Nantucket, Massachusetts. Female, adult. Mus. Comp. Zoöl. No. 1727. Lateral aspect. ¼ nat. size. Tip of beak, left pterygoid and malar defective. 2. _Mesoplodon densirostris_? Skull. Annisquam, Massachusetts. Female, young. Boston Soc. Nat. Hist. Lateral aspect. ¼ nat. size. Distal portion of beak defective and warped. [Illustration: Plate 8 SKULLS OF MESOPLODON EUROPÆUS] Fig. 1. _Mesoplodon europæus._ Skull. Atlantic City, New Jersey. Male, young. Cat. No. 23346, U.S.N.M. Lateral aspect. About ¼ nat. size. 2. _Mesoplodon europæus._ Skull. North Long Branch, New Jersey. Female, adult. Mus. Comp. Zoöl. Lateral aspect. About ¼ nat. size. Distal portion of beak lacking. [Illustration: Plate 9 SKULLS OF MESOPLODON STEJNEGERI] Fig. 1. _Mesoplodon stejnegeri._ Type-skull. Bering Island. Immature. Cat. No. 21112, U.S.N.M. Lateral aspect. About ¼ nat. size. Premaxillæ, maxillæ, frontals, zygomatic process, etc., defective. On account of these defects and the immaturity of the individual the forward inclination of the supraoccipital is much greater than in the skull shown in fig. 2. 2. _Mesoplodon stejnegeri._ Skull. Yaquina Bay, Oregon. Adult. Cat. No. 143132, U.S.N.M. Lateral aspect. ¼ nat. size. Proximal end of premaxillæ defective. [Illustration: Plate 10 SKULLS OF MESOPLODON] Skulls of _Mesoplodon_. Fig. 1. _Mesoplodon bidens._ Nantucket, Massachusetts.