The Oregon skull exhibits all the characters included in the original diagnosis of the species,[31] but two of these, namely, the lack of a groove in front of the premaxillary foramen, and the vertical position of the premaxillæ distally, I do not at present consider of any importance, as they are shared by _M. bidens_. The species, as represented by the Oregon skull, however, presents other characters which clearly differentiate it from any other species of the genius. As it is without a basirostral groove, it allies itself in that respect to _M. bidens_, _europæus_, and _hectori_. Unlike those species, it has the premaxillary foramen behind the maxillary foramen, and in this respect resembles _densirostris_ and _grayi_. Perhaps the most salient characters in which _stejnegeri_ differs from _bidens_ and all other known species are the erect position and flat surface of the supraoccipital and the very prominent backward extension of the frontal plate of the maxilla. This backward extension is so great that when the beak is horizontal a vertical line through the posterior margin of the maxilla passes considerably behind the temporal fossa. The only species which approaches _stejnegeri_ in this respect is _hectori_, but in the latter the supraoccipital instead of being flat above the condyles is very strongly convex. Another very marked character of _stejnegeri_ is that the extension of the lateral free margin of the orbital plate of the frontal, anterior to the orbit, is equal to the length of the orbit itself. In _bidens_ and all other known species this extension is only from one-third to one-half the length of the orbit. Numerous other distinguishing characters will be mentioned in the course of the following description of _stejnegeri_, which is drawn from the adult Oregon skull, but modified when necessary by reference to the type skull from Bering Island. Comparisons are made chiefly with _M. bidens_, which is on the whole the best known species. In the Oregon skull of _stejnegeri_, the breadth between the post-orbital processes does not exceed the length from the occipital condyles to the maxillary notches. The skull is, therefore, narrower in proportion to its length than in any other species of the genus except _hectori_, as represented by the skull figured by Flower. This skull was, however, that of a young individual. It is probable that in adults of this species the skull is broader than in _stejnegeri_. In the latter species, again, the length of the brain-case, between the occipital condyles and the maxillary notches, is just equal to the distance from the latter point to the distal end of the maxillæ, and the rostrum, including the premaxillæ, is much shorter than in other species of _Mesoplodon_, except _hectori_, as represented by the young skull above mentioned.