DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 257 
VIL). Then comes the arched part containing the anterior canal (a.s.c.) bent over 
the hollow for the flocculus cerebelli ; the arch runs backwards and inwards to meet 
the occipital arch, strongly clamped by the thick hollow squamosal (sqg.). From the 
bridge over the facial nerve there is on the left side a short, and on the right side a 
long, spur of bone, running upwards towards the arch of the anterior canal. The 
whole auditory capsule is well ossified, just a little cartilage remaining in the mastoid 
eminence (op.) and along the epiotic margin, but the occipital arch has still large 
synchondroses (Plate 37, figs. 1-3; Plate 38, fig. 1; and Plate 39, figs. 1, 2). 
The supraoccipital (Plate 37, fig. 3, s.o.) is a large shield of bone, round above, 
notched below, and with sinuous sides ; it was, evidently, double at first, is convex 
above, and aloug the middle, and is a little scooped for muscular attachment infero- 
laterally. Then comes a large triradiate tract of unossified cartilage, below which 
there is, outside, the mastoid bone (op.), and inside, the exoccipital (¢.0.); the former 
ends in a small cap of cartilage. The curved exoccipitals are everted against the 
mastoid bone, but they keep well inside it, and form no definite paroccipital pro- 
minence. The condyles are long (deep) and roughly creseentic—broad above, narrow 
below 
and help to enclose a large foramen magnum (/:m.). 
The basioccipital (Plate 39, figs. 1, 2, b.0.) is twice as long as the basisphenoid, it is 
narrow in front, broad behind, subconcave above, and subcarinate below ; it is 
separated from the lateral ossifications by a large tract of cartilage, aud is perforated 
by the large hypoglossals (XII.), and notched by the vagus and glossopharyngeal 
(x, EX): 
So far advanced is the embryo before birth—equal to that of a young Marsupial 
eight months old—and the Marsupials ossify their skull earliest of all kinds—that the 
processus gracilis of the malleus (Plate 37, fig. 4) is already only half the size of the 
manubrium (mb.). The posterior angular process (p.cg.) is a distinct knob, but 
small; the head of the bone is bulbous. The incus (7.) is very bulbous, and is very 
similar to that of a very different Order of Herbivores, namely, the Ruminantia 
(Doran, plate 61, figs. 12-21). The short crus (s.¢.7.) is very short and small, and the 
long crus (/.c.7.) although normal in form, is small as compared with the bulbous 
body. The height and basal breadth of the stapes (sf.) are about equal, the fenestra 
is small, as in those Marsupials that have this hole ; the interhyal is arrested. 
The ceratohyal (Plate 38, fig. 3, chy.) is in one slender subfalcate segment, with 
the lower part ossitied; the hypohyal (h.Ay.) is less than half of the length of the 
ceratohyal, but it is thicker; it is not ossified. Nor is the U-shaped basal cartilage 
(bh.br.) which has flat and uncinate thyrohyals (t./y.), not yet segmented from the basal 
tract, which is small and narrow. This os hyoides is quite unlike the perfect arch of the 
British Insectivora and the East African Rhynchocyon ; it is better developed than 
that of the larger Centetidee, and is about equal to that of a Marsupial at the same 
stage of development. The small, simple concha (Plate 37, fig. 5), shows an imperfect 
segmentation of the annuli inside the dilated leafy growth of cartilage. 
MDCCCLXXXV. 24 
