DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
77 
the latter bone from the great keystone piece ( s.o .), which is notched in a triangular 
manner for the upper angle of the exoccipital. 
The rounded, projecting, rather obtuse angle of the great supraoccipital is not in 
the middle, but below it, for the bony section along the top is one-fourth longer than 
that which leans over towards the spine, behind. Here, as in the Monotremes, the 
hind brain lies in a great hollow recess, formed by the ossified chondrocranium, and 
does not simply lean against a moderately concave wall. If the fore angle of the 
supraoccipital had been sharply pointed, and the frontals more developed backwards, 
we might have had what is seen in the highest physoclystic Teleostei, namely, lateral 
parietals, and the supraoccipital meeting the frontals. Here, the cut edge of the 
occipital “ tegmen ” is longer than the inner edge of the parietal, where it meets its 
fellow to form the sagittal suture. 
The tilting of the auditory capsules has been so great that their hind border 
lies almost flat upon the exoccipitals and basioccipital. The capsule is rather small, 
well ossified, and from the smallness of the space, and its immaturity, the form oi 
the exquisite labyrinth within has been retained in the bony capsule. The anterior 
canal, with its ampulla (a.s.c.), is seen arching over a very definite recess for the 
flocculus; and also its junction with the posterior canal (p.s.c.). Over both these 
canals the bony capsule becomes angular, to fit into the interspace of the neigh¬ 
bouring bones. Across, below the swelling of the vestibule, where the ampullae open, 
the archway of the meatus interims shows the normal passages (VII., VIII.) and, 
antero-inferiorly, below these, the coils of the cochlea. 
A part of the exposed lower surface (fig. 9, chi.) shows a very tumid proximal coil, with 
no definite mark of the further turns of the helix. This figure clearly shows how the 
squamosal, where it opens into the air cavity of the drum, helps the “ tegmen,” 
forming an eave over it, and that under this eave the top of the 1st and 2nd visceral 
arches are sheltered. The secondary fenestra ovalis, and the primary fenestra rotunda 
(inside si., and fr are here shown, the former closed by the proximal part of the 
hyoid arch, or stapes ( st .). The sub-proximal part of that arch, the epihyal ( e.hy .), 
is confluent with the opisthotic region (op.), and has formed the bony “tympano- 
hyal” of Flower —the proximal endoskeletal part of the so-called “stylohyal.” 
The remainder of the hyoid arch is shown separately (fig. 6); the ceratohyal (e.hy.), 
as in the Sloth, is undivided, and is followed by a longish hypohyal (h.hy.); this 
is pointed at its distal end, and joined, lightly, to the basal piece; it is slightly 
ossified. The ceratohyal is largely ossified; flattened above, and united, there, by 
fibrous tissue to the epihyal; below it is thick, and presents a flat face to the flat face 
of the hypohyal, where there is a joint cavity. 
The basihyobranchial (b.h.br.) is U-shaped ; the median part is a thick wedge of 
cartilage, the diverging horns are solid, round, bony rods, tipped with cartilage—the 
thyrohyals ( t.liy.). 
* According to the excellent researches of Professor Alexander Fraser. 
