The foramen magnum is very small, being less in width than the condyle on either side of it. In this respect it differs widely from bidens and other species (as far as can be ascertained from the figures available), except europæus, in which the relative size is about the same.
The supraoccipital rises vertically above each condyle to the very top of the skull, being neither convex nor strongly bent forward as in other species, and especially bidens. In the median line, however, while the occipital bone is flat immediately above the foramen magnum, it is deeply concave higher up and without a median ridge. The outline of the occipital crest, viewed from behind, is semicircular. In all the foregoing characters the occipital region differs widely from that of bidens and other species. The only close resemblance is found in the old skull of europæus from Long Branch, New Jersey, and even here the sides of the occipital above are far less prominent, their outline is much more convex, the occipital crest is angular, and the median depression is less pronounced.
Dorsal aspect ([Pl. 3], figs. 1, 2).—The most noticeable feature of the upper surface of the skull is the large backward extension of the frontal plates of the maxillæ, the free margins of which converge strongly. The outline of the anteorbital region is rounded. The anteorbital notch is a shallow emargination. Anterior to this is a second still shallower emargination, the “pseudo-notch.” The margin between the two is much thickened, but does not form a distinct projection or tubercle, as in bidens and other species. The superior orifices of the nares are unsymmetrical as regards position, the left being somewhat in advance of the right. The maxillæ are concave around the maxillary foramen, and external to this foramen is an elongated ridge about as in europæus. The rostral portion of the maxillæ is broad at the base but tapers more rapidly than in bidens. The margin is thick. At the middle of the beak the outline of the maxillæ at a lower level is visible from above, which is not the case in bidens or europæus. The rostral portion of the premaxillæ is oblique proximally and vertical distally. Unlike bidens, these edges are sharp throughout. The mesethmoid ends opposite the maxillary foramina. Anterior to it is seen the concave upper surface of the vomer, which, however, becomes flat distally. At about the middle of the beak the anterior end is clasped by the posterior forked end of a “mesirostral” ossification, which has a convex surface. This ossification begins proximally below the edges of the premaxillæ, but its surface rises gradually anteriorly, and at the end of the beak it is much above the premaxillæ. The end of the beak consists of the consolidated mass of the premaxillæ and mesirostral ossification, the whole being convex above and below, but flat on the sides. The ossification has a deep median groove, which reaches to within 95 mm. of the tip of the beak.
It will be seen that the conformation of the upper surface of the beak is quite different from that of bidens or any other species.
The maxillary foramina are large and directed forward, and have a distinct broad channel in front of them. In the Oregon skull the right foramen is single, but the left divided into two. The premaxillary foramina are a little behind the maxillary foramina. The distance between the maxillary foramina is less than that from the median line to the anteorbital notch. In bidens it is much greater.
Lateral aspect ([Pl. 9], figs. 1, 2).—A most noteworthy feature of the skull when viewed from the side is the great length between the orbit and the maxillary notch, which far exceeds that found in bidens and other species, being equal to the length of the orbit itself. The latter is about as long as the temporal fossa, which is somewhat flattened above, as in europæus. The outline of the supraoccipital is straight and nearly vertical. The zygomatic is more massive even than europæus and is especially thick below. The inferior outline of the beak is convex proximally as in europæus and layardi. There is no basirostral groove, the edges of the maxillæ being very thick in front of the maxillary notch. Over the orbit the maxillæ are thick and beveled, but not raised as in bowdoini.
Ventral aspect ([Pl. 6], figs. 1, 2).—The beak is convex in the proximal half, much as in europæus, but farther forward is concave, except in the median line, where there is a narrow ridge formed proximally by the vomer, which in the type skull appears as a narrow lozenge 60 mm. long. In the adult Oregon skull it is anchylosed with the premaxillæ. The maxillæ extend to within 107 mm. of the end of the beak. The under surface of the beak is much more like that of europæus than of bidens.
A narrow strip of the palatines extends around the base of the pterygoids in front, but the two strips do not meet in the median line. In the type-skull they do not extend inside the pterygoids. The expanded anterior end of the malar is very long and also forms the bottom of the maxillary notch, which is the case in europæus but not in bidens. The inferior borders of the pterygoids are convex anteriorly, as in europæus, and are continued laterally, so that the sinus is deep as in that species. The lachrymal is very long, the free margin having a length of 55 mm. The posterior margin of the zygomatic process is concave, rather than convex as in bidens.
The tympanic bulla does not differ materially from that of bidens in size or shape, as far as can be judged from the figures given in Van Beneden and Gervais’ Osteography (plate 26, figs. 4, 4a). The periotic is similar in size to the same bone in bidens, but the posterior end is more narrowly pointed and the anterior end is much lower, relatively. In europæus, as far as can be determined from the material at hand, the form and size of the earbone is similar to that of stejnegeri, but in the latter the anterior margin of the tympanic bulla is more nearly transverse and the posterior inferior groove is curved. ([Pl. 35], fig. 2.)
In the Annisquam skull, supposed to represent densirostris, although from a young individual, the earbone is very much larger, especially the periotic, which is also quite differently shaped.