DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
205 
fig. 7, o.s.) is now much lessened (Plate 31. fig. 11, o.s.) ; its posterior band has been 
absorbed, leaving only a point to the hind corner of the wing; the hind edge is now 
ossified as the narrow orbitosphenoidal centre. 
The oblique, notched, anteriorly-placed alisphenoid (al.s.) is only partly seen in this 
view, because of its great out-thrust; its base ( b.s .) and the basioccipital ( b.o .) are seen 
to be rather thick plates of bone. The other elements of the occipital arch ( s.o ., e.o .) 
are set in a crescent, whose convexity looks backwards; all the synchondroses are 
large, as yet. 
There are only two periotic bony centres—the prootic ( pr.o'.) which forms the 
curious wall-plate, and the opisthotic, which builds-in the labyrinth. 
The mastoid region behind, enclosing the posterior canal (p.s.c.), and the arch of the 
anterior canal (a.s.c.) are still invested with cartilage, but the common sinus, behind 
the cerebellar fossa, has a bony inner wall. Everything in this skull, as in that 
of the Mole, conforms to the dominant idea of a skull to be manipulated, so to speak, 
into a smooth, elongated borer, so that it may resemble the head of a Weevil ( Curculio ). 
Thus all the investing bones are thin, gently convex, and so imbricated as to thrust 
out no projecting points, externally. 
This section shows the nasal, frontal, parietal, and interparietal (n.,f, p., i.p.) above ; 
and the premaxillary, maxillary, palatine, pterygoid, and tympanic (px., mx., pa ., prj ., 
a.ty.), below. Infero-laterally, the trough-shaped squamosal (sq.) is seen from the 
inside as a narrow tract, arched above, but not reaching the rounded lower edge of the 
parietal, and the concave edge of the latter fails to reach the arched top of the prootic 
plate, thus there is a considerable lateral fontanelle or part formed of membrane only. 
Near the lower edge of the parietal, the lateral sinus (s.c.) throws its elegant arch 
over the inside of the skull, from the auditory capsule to the orbitosphenoid. 
When the endocranium is seen divested of nearly all its investing bones, from 
below (Plate 29, fig. 4), we have what seems to be a remarkably imperfect skull, even 
considered as a basin, and not as a box. What we see is a remarkable result of 
dwarfing and elongation; there is a thrifty use of every kind of skeletal material, and 
the substances used have been thinned out as far as could be done safely. Yet the 
skull of a Child at the same stage, similarly prepared, would show the same elements 
disposed in a similar manner, and there would be little difficulty in recognising most 
of the homologous parts. 
The conchoidal narial region (e.n., al.n.), right and left, and the fluted double nasal 
tube, are seen to form a very perfect cartilaginous structure up to the part where the 
dentary region of the premaxillaries is attached. There the floor is cut away in a 
rounded manner, and the nasal labyrinth is open below, almost to its end, where its 
moieties do close in, independently of each other, right and left of the forks of the 
vomer ( al.e ., v.). This open region begins by forming the long retral tracts that help 
to encapsule Jacobson’s organs (figs. 4, 5, rc.c.,j.o.) ; there is no Mammal in which I 
have, as yet, been able more satisfactorily to work out these parts than in this young 
