DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
107 
zontal canal ( h.s.c.); behind this the larger crescentic swelling is due to the posterior 
canal ( p.s.c.). The exoccipitals ( e.o.) margin the foramen magnum (see also fig. 4, 
e.o.); below, the occipital condyle ( oc.c .) is shown, and outside this the low par- 
occipital ridge ( p.oc.). 
The facial nerve (VII.) is seen escaping below the horizontal and posterior canals 
where they meet, and in front of the nerve the epihyal ( e.ity .), with its obliquely 
oblong splint, the stylohyal ( st.li .). The cartilaginous bar is elbowed and enlarged; 
it then runs downwards and forwards into the upper ceratohyal ( c.hy.) without 
segmentation. Then comes the lower ceratohyal half as long as the main bar, and 
then, less than half as long as the lower ceratohyal, the hypohyal (h.hy.). That 
little hillock of cartilage dilates to articulate with the fore part of the U-shaped 
basal bar ( b.h.br .), a very regular semi-ellipse. The sides (or horns) of which are the 
continuous thyrohyals ( tjiy .). 
When the hyoid bar becomes segmented from the cranial part, then this will be 
quite a normal hyoid arch, except that the thyrohyals are continuous. But in the 
Edentata the hyoid arch is most variable, and that not only as to Families, but the 
genera Dasypus and Tatusia do not agree in the structure of this arch. 
The smallness and imperfection of the teeth has given rise to a great reduction in 
the size and bulk of the fore part of the mandible; this is a much larger “ramus” 
than that of Tatusia (Plate 6, fig. 3), for the hind part is very large, the coronoid 
process' ( cr.p .) being of great extent and rather wide. The articular or condyloid 
part ( cd.p .), with its oval condyle, runs downwards and forwards till it meets the over¬ 
lapping thick part where the vessels and nerves enter. That thickening corre¬ 
sponds to the distinct coronoid bone of the Ovipara, and the gently convex inner wall 
of the ramus to the splenial bone. Below the foramen, the ramus forms a second 
lobe similar to the rounded angular process (ag.p.) behind. 
The end view (Plate 14, fig. 4) shows the occipital plane, still largely unossifiecl. 
The investing bones (i.p ., p ., sq.) can be seen overlapping the whole of this hind wall, 
and the auditory capsules (op.), swollen here with the posterior canal ( p.s.c .), are seen 
planted into a deep oblique notch, right and left; below these the facial nerve (VII.) 
and the epihyal ( e.liy .) come into view, The rest is occipital, and here we see, as a 
correlate of the development of the large intraparietal, a decrease in the size of the 
subcircular supraoccipital ( s.o .); see also in Tatusia (Plate 6, fig. 4). By this we 
know that we are further from the Monotremes, and from the rest of the Edentata. 
A large sinuously transverse muscular ridge runs across this bone already, so that here, 
as in the temporal region and lower jaw, we see that we have to do with a strong and 
muscular animal. The subcircular foramen magnum (fan.) has the outer part of 
its arch and upper half occupied by the rounded exoccipitals (e.o.) ; their depth, the 
depth of the cartilage between them and the supraoccipital centre, and the depth of 
the occipital condyles (oc.c.), are all nearly equal. Outside the condyles, the arch 
projects as a quite distinct, though not large, paroccipital ridge and process (p.oc.) : a 
P 2 
