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.
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, 4_a_). 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.