DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 299 
Here the zygomatic process, the glenoid cavity, the postglenoid ridge, the supra- 
tympanic notch, the temporal fossa, the tempora squama, and the post-tympanic 
angle are all in a high state of development. Below and inside the elegant sigmoid 
jugo-squamosal suture we see the internal pterygoid hook, and the small external ptery- 
goid plate (pg., e.pg.). Built well into the deep rounded tympanic notch of the squa- 
mosal, we see the strong compressed bony meatus externus (m.t.c.), standing out from the 
general tympanic dish (a.ty.) ; it is open above, as the squamosal is below ; together 
they make a vertical narrow bony-lipped passage to the shallow drum-cavity and its 
extensive upper galleries—that is, after the membrana tympani has been removed 
from the inside of this narrow porch. The lower jaw (figs. 4 and 5) has, already, 
nearly assumed the adult form, with its deflected mentum, its oblong dentary region, 
its small unciform coronoid process, its non-pedunculate condyle, notched behind, and 
its wide out-turned angular region, with an incurved inner edge. 
The lower view (Plate 37, fig. 6) shows some contrasts with that of the embryo of 
the other species (fig. 1) that cannot all be put down to a further stage of development. 
Here we must make allowance for two causes of difference—age, and diversity of 
species. * 
Here (Plate 37, fig. 6) the alveolar tracts are more nearly parallel than in the 
other kind (fig. 1), and this is not merely due to advance in growth, but is partly 
also specific. The fore part of the premaxillaries (pz.) is slenderer, and the maxillary 
palatine plates have grown further forwards. 
With this fuller development of the palatine plates, the hollowness of the palate in 
its middle—a remnant of the old Marsupial feebleness of this part—is almost gone, 
although the mid-line is hollow, and a shallow fossa runs right and left, cross-wise, 
near the hind margin of the maxillary plates. The palatine plates of the palatines 
(pa.) are like those of the other kind, but they are more extensive, and in this stage we 
have a complete sutural analysis of the peculiarly elegant archways of the nasopala- 
tine passages of the adult Colugo. The squamous suture of the maxillaries and 
palatines is arched, but runs transversely in this case, and the narrow fissure between 
the flat part and the ribbed edge is widened out into a very remarkable uncinate con- 
cavity, ending sharply, outside, in the posterior palatine foramen (p.p-). Here, 
again, we see the sharp angular space between the palatine part and the mere side 
wall, which turns inwards to join the feeble pterygoids (pg.), which have a basicranial 
tract, ending in a toothed edge, between the orbital plates of the palatines and the 
long tongue-shaped mesopterygoids (i.s.pg.). These latter reach nearly to the basi- 
occipital (b.0.). 
* The difference is as remarkable as the similarity (Plate 37, figs. 1 and 6); and if these two wazs can 
show such curious and notable modifications, although so close akin, there can be no difficulty in 
imagining a large number of similar but greater differences in structure in whole hosts of related forms, 
which may at one time have existed, and which were, probably, potent in species, genera, and even sub- 
families. 
OR 16 0% 
