DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 189 
investing bones is now seen to be that of the one hehind on the one before it ; had 
they taken up more of the ewtis vera they would have differed but little from the 
bony scutes on a fish’s head. Below, there is now a more evident angle where the 
premaxillary meets the maxillary ; in this latter bone the infraorbital foramen (V*.) is 
still unfinished. It is covered over, in some degree, in the perfect skull, by the jugal 
(figs. 5, 6, 7.). The frontals (f) have grown down well inside the orbit, but they 
are singularly free from the usual orbital processes; the upper part there forms a 
rounded eave over the retiring orbital plate. That plate has a very large oval 
foramen for the ophthalmic nerve (V!.), and a lesser hole behind that in the extreme 
part. The gap in the wall of the skull seen in the last (fig. 3, fo’.) is here shown as 
filled up by the small oblique squamosal (sq.), the thin, sinuous upper edge of which 
overlaps the antero-inferior part of the large parietal (see also fig. 3). There are 
three sharp processes growing from the slanting lower edge of the squamosal. The 
first is its own front corner, overlapping both parietal and frontal; the next is the 
small, sharp jugal process; and the third is the postglenoid process. The hinder 
swollen part over the auditory region is bilobate; the temporal fossa is extremely 
small and ill-defined Over the roof, behind, the interparietal (7.p.) has grown so as 
to be, relatively, the largest, or nearly so, to be seen in the whole class ; antero- 
posteriorly, it is only one-third the extent of the parietal, but seen cross-wise, or at 
the side, its extent is very large; it nearly reaches down to the exoccipital (¢.0.). 
Below, the palatine (pa.) is seen a little, and the pterygoid (pg.) better; this has 
still a nodule of cartilage on its hook (pg.c.). Under the very perfect system of 
investing bones—which serve the purposes of “ashlar” in a building—the almost 
completely ossified endocranium is partly seen. The orbitosphenoid (0.s.) does not 
show its optic foramen well in this view (see fig. 8, 0.s., II.), but the sphenoidal 
fissure, and the foramen rotundum and foramen ovale in the alisphenoid (al.s., 
V*., V°.) are displayed in this aspect. Under the alisphenoid, the broad, cellular 
tympanic wing of the basisphenoid (¢.b.s.) is quite visible in this side view. A 
remarkable amount of the very large skeleton of the auditory capsule is seen in this 
figure. In the somewhat obtuse angle between the parietal and interparietals 
(p., ip.) the polygono-ovoidal prootic wing (pr.o’.) is almost entirely displayed ; it is 
very convex, and, if shorter, is much broader than the squamosal, for which it might 
be taken in a cursory view of the adult skull. The opening from the lateral sinus is 
behind the prootic wall-piece ; under that bone there is a large tongue-shaped epiotic 
tract (ep.), it is not a distinct centre, but merely a lobe of the opisthotic (op.).* Under 
it there is a foramen, above the large passage for the vagus and glassopharyngeal 
nerves (X., [X.). The opisthotic region of the opisthotic bone, the proper “ mastoid ” 
tract, is an irregular wedge of bone running from the post-temporal part of the 
squamosal to the exoccipital (e.0.), with its almost suppressed paroccipital ridge ; that 
bone is, like the mastoid region of the opisthotic, an oblique, irregularly-oblong tract. 
* There is a deficiency in the lines of reference to these parts in this figure. 
