manner from the external basal line of the crest, being most strongly marked in the
constriction; hut they disappear in the adult, in which the external surface is
coarsely rugose. At all ages, the anterior two-thirds of the inner margins of the crest
have a number of imperfections of ossification which communicate with the vacuity
between the two plates; these also extend along the anterior border, but do not
occur in the posterior third of the margin, as the internal plate is imperfect in that
portion of the crest. A number of foramina open behind the anterior border. The
anterior and* middle thirds of the inner borders of the crests approach each other
with age, hut never touch; while their posterior thirds are strongly divergent from
within outwards, defining a triangular space and constituting the osseous limits
of the blow-hole. The right margin of this space is longer than the left, owing to
the sinistral asymmetery of the skull.
The premaxillary surface is a very narrow area lying along the anterior base
of . the crest, above the posterior orifice or termination of the cavity of the dental
portion of the jaw, and at its posterior half is internal to the fronto-nasal surface.
I t has an outward and inward curve from above downwards and is completely
invested by the premaxilla. The outer margin of the premaxillary portion forms
a rather prominent ridge. The fronto-nasal portion narrows from above downwards
between the inner basal margin of the crest and premaxillary surface; on
the right side, it is concave on its upper half, and slightly convex on its lower, while
on the left side these characters are reversed. A little below the superior extremity
of the premaxilla, and immediately external to its outer border, a foramen occurs in
the upper half of the fronto-facial portion and leads nearly vertically downwards to
a canal that opens below at the outer border of the palatine, but anterior to it in the
upper portion of the spheno-maxillary fossa, defined by the palatine. This canal
transmits a nasal branch of the fifth nerve.
The anterior aspect of the crest, looking inwards over the nares, consists, in its
lower half, of a thin plate of bone corresponding to the flat perforated surface of the
external aspect of the maxilla. In this surface, the two plates of the maxilla appear
to be lost in each other, where they meet below the orbit, and the external plate of the
crest thus appears to be quite distinct from either of them, these amalgamated plates
forming the inner plate exclusively. This plate is very thin and is marked by a
number of imperfections of ossification in its lower portion; but after passing over
the lower half of the anterior or nasal surface of the crest, the imperfections of ossification
become enormous, and the appearance is produced as if the plate stopped short
at this point. But, from its anterior portion, a narrower surface is prolonged round
the free margins of the crest to near its posterior superior angle; between this and
the body of the plate pass numerous branched osseous spicules connecting the
plate and its marginal portion together, the interspaces between the connecting
bands being imperfections of ossification. In adults, the inner plate becomes much
thickened and consolidated in its lower portion, and the narrow border, along the
margin of the crest, also becomes much thickened and detached from the former,
by absorption of the osseous connecting bands.
Sexual differences displayed by maxilla/ries.—The characters which distinguish
the snouts of the males and females are illustrated by the skulls represented in Plate
XXXIX, and I have already pointed out what these characters are«
Changes by age in the alveoli and dentition.—The alveolar surfaces lie side by side
throughout their extent, except posteriorly, where they are divergent at the fourth
tooth from the last. In very young skulls the alveolar furrow posteriorly, as far
forwards as the eighth or ninth tooth from the last, is continuous with the cavity of the
maxilla, but about these teeth a narrow ridge appears on the inside of its external
wall. This ridge soon re-unites with its inner wall, constituting a distinct roof perforated
by foramina, which separate the furrow from the cavity of the jaw, throughout,
however, only a very limited part of its extent, as this septum ceases about the
twentieth tooth from the hindmost, the jaw cavity and furrow being merged in one.
The formation of alveolar pits I have also previously described Under Dentition.
Premaxilla (PI. XXXIX, figs. 1 and 2, and PI. XL, figs. 17 and 18).—This
bone is nearly of the same length as the maxilla and consists of two portions,
a facio-nasal plate, almost crescentic in form, constituting the outer border of the
external nares, and a long thin compressed rod continued forward beyond the extremity
of the maxilla. The former plate overlaps the facio-nasal portion of the maxilla, closing
in the posterior termination of the cavity of the jaw and completing the anterior wall
of the nasal canal. The remaining portion of the bone is nearly straight in youth,
but, in adults, it partakes in the same marked curves that distinguish the maxilla. The
outer surface of the premaxilla, about the beginning of the furrow lodging the infra,-
orbital vessels and nerves, is marked by two or three imperfections of ossification in some
skulls, but not in others. The premaxillary foramen occurs about one inch from the
end of the snout in adult skulls, and is close to the upper border which is slightly
divergent from its fellow, displaying the termination of the cartilaginous vomer.
As already stated, the premaxilla appears to carry four teeth, as the line of suture
can be traced along to the base of the fourth tooth; union, however, of the anterior
extremity of the bone with the maxilla is an event of uterine life. The inner
surface is divided into two portions, an upper deeply concave portion lodging the
mesethmoid cartilage, and a thin plate below this, which gradually increases in depth
from before backwards, applied to the inside of the vertical or palatine plate of
the maxilla.
This bone also partakes of the general asymmetry of the skull, its left facio-
nasal, portion having a considerable sinistral twist, owing to the external nares being
dragged to that side of the skull, a circumstance which also tends to reduce the
upward extension of this part below the level of the bone of its right side.
Nasals.—These two small narrow bones lie in a depression on the lower end
of the nasal ridge of the frontal, above where it is continuous with the mesethmoid
portion of the ridge. The lower extremities form a very faint projection, marking
the superior posterior termination of the nasal septum.
The Mandible and its dentition (PI. XL, fig. 19, and PI. XXXIX, figs. 1
and 2).—The distinguishing feature of the mandible, as is well known, is the very