DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 195 
obliterated ; then the bony sheath, itself, soon afterwards, is absorbed ; these things 
are enigmas to us at present. 
The annulus tympanicus is shown in the young Mole, three inches long, from the 
side (Plate 27, fig. 3, a.ty.); and the curious flat, imperfect ring is figured in several of 
the earlier stages (Plates 26 and 28). In the adult it is wholly blended with the skull 
floor, but during the first summer it remains distinct; that of the right side, in 
this stage, is shown from its inner face (Plate 25, fig. 11); it looks like a well-made 
annulus that has been crushed into a flat coiled band of bone. The outer opening 
is roughly pyriform, quite pointed on the inner or lower side, and broad above, and 
having its margins dentated. The part nearest the opening, outside, is very thin; then 
the bone thickens up to the margin ; on the inner face the edge is seen to be strongly 
ribbed by bony deposits. The anterior crus is very large, and grows along the 
inside as a large rostral plate, looking backwards, and nearly meeting the sharp, 
inturned, hinder crus. Thus the great inner opening, which is thrice the size of the 
outer, is also transverse to it; it is a nearly finished oval, with the long diameter 
at right angles with that of the outer opening. The great rostral process of the 
front crus is grooved on the inside, outside its arched edge, for the still large 
processes gracilis of the malleus (Plate 25, figs. 4, 5, p.yr.). 
Thirteenth Stage.—Skull of the adult Mole. 
I could have wished to have given figures fully illustrating the structure of the 
skull in the adult Mole, but considerations of space have deterred me; I have, 
however, figured the ossicula auditis (Plate 25, figs. 8-10), and the snout, in section 
(Plate 25, fig. 14). This latter part has been affected by the generally intense ossifi- 
cation of the skull, and not only the proper septum nasi (s.n.) has been well ossified 
on from the perpendicular ethmoid, but that fore-growth of the septum (s.7.) which 
divides the long alinasal region (a/.n.), in front of the premaxillaries, has acquired an 
endosteal tract, almost to its front end; the teleology of this structure is evideut 
enough, but it comes in as a part of the general osseous modification of this type, 
which is everywhere intense. 
The prootic plate, and the hinder or mastoid region of the auditory capsule, have 
sutures separating them from the surrounding bones. The parietals, also, are always 
free; they form a squamous suture with the frontals, which they overlap up to the 
middle of the interorbital region, but the true squamous suture, between them and 
the squamosals, is very slight ; they keep their own mutual saggital suture perfect. 
With these exceptions the skull of a Mole has its bones as completely anchylosed 
together as in any Bird; and the structure of the base of the skull is as light and 
pneumatic as anything seen in any Passerine, or highest, kind of Bird. During the 
first summer there is but little promise of the exquisite polish of the bones nor of the 
excavated condition. they will soon attain to. Everything becomes smoothed down, 
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