The type-specimen of _Ziphius semijunctus_ (Cope), as already mentioned, is a young female.[38] The most noticeable characters which it presents are that the premaxillæ are flat proximally, and that the teeth are small, sharp-pointed and open at the roots. The form of the teeth is undoubtedly due to immaturity, but as the shape of the premaxillæ is similar to that found in the nominal species _gervaisii_, it might be thought necessary to refer _semijunctus_ to the latter species. As will be shown later, however, this form of the premaxillæ appears to be characteristic of the adult female of _cavirostris_, and of immature individuals of either sex, the young, as in many kinds of animals, resembling the adult female rather than the male. I have been able to find but one character in the skull of _semijunctus_ which might be regarded as specific. This is that the lachrymal bone is thick distally, and cut off square at the end. In other specimens of _Ziphius_ examined it is thin and flat, and rounded or pointed at the end. As there is much individual variation in the form of the lachrymal, this peculiarity alone is, in my opinion, an insufficient indication of the validity of the species. COMPARISON OF SKELETONS. A comparison of the skeletons of the three individuals from the Atlantic coast of the United States reveals a number of differences of more or less importance. Were it not for the lack of reliable differences in the skulls, it might be considered that these variations in other parts of the skeletons indicated specific difference. I am disposed, however, since the Barnegat and Newport specimens are of opposite sexes, to regard them partly as sexual and partly as individual. In the case of the Charleston specimen (_semijunctus_), the skeleton, besides being immature, has been very much damaged by careless handling, and nearly all the bones are somewhat abraded. It is, therefore, only available to a limited extent for purposes of comparison. As no description of a _Ziphius_ skeleton from the coast of the United States has, so far as I am aware, been published hitherto, and as descriptions of skeletons of Old World specimens are few and rather brief, I shall give below a detailed comparative description of the American specimens. For the sake of brevity, I shall refer to each specimen merely by the locality. VERTEBRAL COLUMN AS A WHOLE. The vertebral formula in the three North American specimens and in four Old World specimens and Burmeister’s Argentine specimen is as follows: _Vertebral formula of Ziphius cavirostris._