The chief differences between the Barnegat City and Newport skeletons are in the size and form of the processes of the cervical vertebræ, the form of the seventh and eighth thoracic vertebræ and of the ribs connected with them, the direction of the acromion of the scapula, the shape of the first phalange of the first digit, and of the posterior segments of the sternum. As far as the processes of the cervicals are concerned, these are known to be extremely variable in all cetaceans. The seventh and eighth thoracic vertebræ are those on which the mode of attachment of the ribs changes in ziphioid whales, and I have observed in the genus _Mesoplodon,_ as here, that the processes and articular facets were very variable, being sometimes quite unlike on the two sides of the same vertebra. The direction of the acromion is probably subject to large individual variations, though this can not be determined at present, and the same is true of the form of the first phalange of the first digit. The form of the sternum is quite variable in all cetaceans, and can not be relied on for specific characters, without comparison of many individuals. On the whole, I am of the opinion, as already stated, that we are not compelled by the differences noted to regard the Barnegat and Newport skeletons as representing different species. The Charleston skeleton is too young and imperfect to admit of serious consideration. The idea that the differences between the adult skeletons are probably individual receives support from the fact that the skeleton shown in the photograph from St. Simon Island, Georgia, mentioned on page 31, No. 14, appears to possess a combination of characters exhibited by the other two. AGE VARIATIONS IN SKULLS. The series of skulls of _Z. grebnitzkii_, which the Museum owes to the activities of Dr. L. Stejneger and Mr. N. Grebnitzki, comprises specimens of different ages, and, as will be shown presently, probably both sexes. Taken together with the skulls from the east coast of the United States they probably represent very fully the variations which the skull undergoes in the present species. These changes may, perhaps, be best made evident by the following brief descriptions of the various skulls: