DEVELOPMENT OP THE SKULL IN THE MAMMALIA. 
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equidistantly, in two places, by the 2nd and 3rd branches of the 5 th nerve, thus 
forming the foramen rotundum, and foramen ovale (V 2 ., V 3 .). The thick semicircular 
inner margin of this outer lobulated part stands off from the main plate, and the 
whole of the outer part lies below, free of, and at a distance from, the orbitosphenoid, 
which it overlaps considerably. This very diagnostic Mammalian alisphenoid is 
followed, postero-externally, by a large fenestra, a space totally devoid of cartilage, 
but which is being filled up by the squamosal bone ; it is the upper part of the 
tympanic space, and is traversed by the ossicula auditus. Round it, like a bow, the 
orbitosphenoidal band ( o.s'.) is bent, passing, behind and above, into the supra-auditory 
cartilage ( s.a.c .); below', this band forms the fore part of the tegmen tympani, and the 
incus articulates, by its short crus, at the junction of this lateral band with the 
auditory capsule. These capsules, in their basicranial setting, are very elegant 
structures; they stretch from the tympanic lobes of the basisphenoid, antero- 
internally, to the feebly - expressed paroccipital ridge, right and left, postero- 
externally. The cochleae (chi.) show their three coils, and the fenestrse rotundse (/.r.); 
these are very large, and well seen from below. 
The fenestra vestibuli is closed by the stapes (st.), a small irregular ring of cartilage. 
Up to the passage for the 9th and 10th nerves (IX., X.) the capsule is very distinct 
from the chondrocranium, but in the mastoid region below the semicircular canals, and 
where the posterior canal is imbedded, there is more or less fusion of these parts 
The very large relative size of the occipital arch reminds one, at once, of that of the 
Echidna; here the chondrocranium is as complete as in the Skate. 
The notochord ( nc .) is seen from below, up to the point where it rises into the post- 
clinoid wall (see fig. 3, nc., p.cl.), in which the proper, primary axis of the animal ends, 
and beyond which everything is of the nature of an outgrowth.'" 
An elegant narrow waist is formed to the basis cranii by the pressure of the large 
cochleae ; behind this part the parachordal tract expands sinuously, and runs upwards 
into the side walls. The whole hind part is very smoothly rounded, and the condyles 
(oc.c.) are very flat, and have a sulcus across them ; the foramen magnum ( f.m .) is 
very large; the f. condyloideum (XII.) is small and far outwards, near the concave 
edge of the arch. 
The upper view (Plate 25, fig. 3) shows the roof of the nasal labyrinth, with its 
long fore part, and its lateral lobular expansions right and left of the deep, multi- 
perforate rhinencephalic recess ( cr.p.). The crested intertrabecula, at its junction 
with the hind part of the nasal roofs, above, shows a small crista galli ( cr.cj .); the wall 
below this part is thin above ; it is the top of the perpendicular ethmoid (p.e.), which 
widens, gently, to pass into the presphenoid ( p.s .). The narrow, backwardly-curved 
* There are two ways of looking at the prochordal tracts of the skull—the trabeculae and inter- 
trabecula ; some see in these parts a highly modified, first visceral arch; I confess that, at present, they 
merely seem to be oagrowths of the proper axis to finish the new, highly expanded fore part of the skull; 
made necessary, in the Yertebrata, by the great expansion, even in the lowest kinds, of the neural axis. 
