DEVELOPMENT OP THE SKULL IN THE MAMMALIA. 
39 
The end view of the skull (fig. 4) shows a very large orbicular supraoccipital ( s.o .), 
divided vertically by two shallow sulci into three convexities. The lateral elevations 
have a large selvedge of cartilage, half their breadth, which has to be ossified by this 
large post-cranial shield of bone. 
The paroccipital ridges are very obscurely marked off from the mastoid region (op.); 
but the exoccipitals ( e.o .) are getting near to the upper centre (s.o.), over the large 
convex condyles (oc.c.) ; these latter are wide apart, and show the basioccipital ( b.o.) 
between them. The parietals, squamosals, and tympanies (p., sq., a.ty.) can be seen, 
end-wise, in this view, as also the epihyal and facial nerve (e.hy., VII.). 
The auditory capsules were removed and figured separately on a larger scale (Plate 
6, figs. 7, 8). 
The inner view (fig. 7) shows a very large squarish part above the bulbous cochlea 
(chi.), and having in it the canals, only the anterior and posterior marking this aspect, 
where they meet in one common sinus. 
The antero-superior part runs forwards as a curved process, which is, indeed, the 
anterior part of the tegmen tympani (see also fig. 11). The meatus internus, 
with its various foramina (VII., VIII.), is very large; another small hole is seen 
under the arch of the anterior canal (a.s.c.), which has no recess for the “flocculus,” 
but only a gentle scooping at this part. Another larger passage is seen, postero- 
inferiorly, inside the stylomastoid foramen (see fig. 8). 
In the outer view (fig. 8) the large square top of the capsule has a sinuous outline 
postero-superiorly and antero-superiorly ; but the mastoid margin, which contains most 
of the posterior canal ( p.s.c .), is straight. It is deeply notched below for the facial 
nerve (VII.), the notch is the stylomastoid foramen, whose lower border is the 
epihyal spur (e.hy.), now separated from the ceratohyal. 
Within and in front of these parts we see the fenestra rotunda (fr.), and at a 
distance equal to its width, the fenestra ovalis (fs.o.). Over these openings the 
tegmen tympani forms an arch, continued forwards in the free spur ; over the tegmen 
the horizontal canal (h.s.c.)- is seen shining through the cartilage. The ossification of 
the capsule is very low and generalised as in certain anurous Amphibia, e.g., Pseudis 
(see Phil. Trans., 1881, Plates 2, 10, 11, 12). The bony matter, which is formed 
endosteally, is showing itself everywhere on the convexity of the cochlea, on the spur 
of the tegmen, along the sinus of the anterior and posterior canals (fig. 7), and on 
part of their arches and their interspace, 
Endocranium oj Tatusia peba. —Sixth Stage (young specimen, apparently new-born, 
4^ inches long). 
These drawings (Plate 6, figs. 9-11) were made from dissections of a large ripe 
embryo or new-born young of Tatusia peba; this, and that figured in Plate 5, 
figs. 5-8, were the only long-headed specimens in my collection; I am satisfied that all 
