Returning now to the question of the validity of grebnitzkii as a species, I would say that after comparing the measurements of the Bering Island skulls with those of the Atlantic coast specimens, and comparing the skulls themselves, I have been unable to find any constant difference of importance, except the size and form of the periotic bone. As the earbones are lacking from many of the skulls, the series available for comparison is small.
As compared with the Atlantic coast specimens, the anterior portion of the periotic bone in grebnitzkii is larger, broader, and more rectangular in outline when viewed from below. I observe, however, that the absolute size and outline of the periotic vary considerably in the different specimens of grebnitzkii without relation to age. The same appears to be true of cavirostris, but comparing the two series of skulls as a whole it appears to be true that the anterior mass of the periotic is larger in grebnitzkii. I do not think, however, that the latter species should be kept distinct on this account alone, at least until the character has been confirmed, and perhaps strengthened by others, through the examination of a larger series of specimens.
SKELETON OF ZIPHIUS FROM BERING ISLAND.
The Museum collection contains an incomplete skeleton of a very young individual, Cat. No. 22875, which was received from Bering Island with the skulls of Z. grebnitzkii, but does not belong to any one of them. Whether it really represents that species is, therefore, uncertain, but such is probably the case. The length of the vertebral column, consisting of 45 vertebræ, without interspaces, is 9 feet 2 inches.
The vertebral formula is as follows: C. 7; Th. 10; L. 10; Ca. 18 (+1?) = 45 (+1?). This is the same as in the type of semijunctus so far as the cervicals, thoracics, and lumbars are concerned, and the probable total is the same. In their general characters these vertebræ agree with those of the skeletons already described, but they present a number of differences as well. On account of immaturity the processes are even less developed than in semijunctus. All the epiphyses are free, and in the third to the seventh thoracic vertebræ the neural arch and spine are separate from the centrum. The centra are very short in proportion to their width.
Although the specimen is so young, the anterior foramen of the atlas is, nevertheless, inclosed by bone, and though the line of separation between the atlas and axis is visible on the sides, the fourth cervical is anchylosed to the third at the top of the centrum. Although the neural spines, metapophyses, and transverse processes of the thoracics are much shorter than those of the young semijunctus, the epiphyses are as large or even larger than in that specimen. The neural arches are also noticeably thicker than in semijunctus, and the centra are rounded inferiorly rather than carinated. The neural spines are much more nearly erect than in the adult Barnegat and Newport skeletons, but, as mentioned on [page 41], this is probably a character of immaturity, and is shared by semijunctus.
The differences as regards the form of the centra and neural arches die away among the lumbars, and these vertebræ and the caudals are, with a due allowance for greater immaturity, very similar to those of semijunctus.
The seventh thoracic is like the sixth in form, and is without a transverse process. It thus resembles the same vertebra in semijunctus. The eighth, however, has an ill-defined facet on the side of the metapophysis and a second facet a little above the upper border of the centrum. The eighth pair of ribs has only a single terminal articular facet.
The ninth thoracic has a short, thick transverse process, about in line with the upper surface of the centrum.
The transverse process of the seventh caudal is perforated on the right side by a foramen. The transverse processes are last traceable on the ninth caudal, the neural spines on the tenth caudal, and the neural arch on the eleventh caudal. Eight chevron bones are preserved, but probably two more were present originally.