DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
41 
the middle of this plate. The thick basisphenoid ( b.s .) is almost as long as wide ; it is 
emarginate, a little, both before and behind. 
The synchondrosis has only half the extent shown in the last stage (fig. 1), and the 
basioocipital ( h.o .) is now a large six-sided bone, having lessened cartilaginous inter¬ 
spaces between it and the exoccipitals [e.o.). These large centres of ossification are 
concave on the inner, and lobate on their outer, side; they are notched, below, by the 
hypoglossal nerve (XII.), and above are separated by a definite cartilaginous tract 
from the large shield-shaped supraoccipital ( v.s., read s.o.). The broadly reniform 
condyles ( oc.c .) are now mere caps of cartilage on the exoccipitals. 
In the detached hyoid arch (fig. 10) the upper and lower ceratohyals and the 
thyrohyals ( c.hy ., t.hy.) are rapidly becoming ossified. 
The auditory region (fig. 11), removed from the rest of the skull, and seen from the 
outside, shows a great increase of the bony deposit, which now runs round the large, 
squarish mass, and is especially developed over the tegmen tympani. The ossicula 
are now ossified; the processus gracilis of the malleus ( pr.g .) is a large spatula, 
with no Meckel’s cartilage left, and lies in a groove in the upper crus of the well- 
developed shell-like annulus tympanicus ( a.ty .). The stout incus (i.) is shown in situ, 
with the head of the stapes seen inside ;.that bone is also shown, detached, and it is 
now high, and narrow, and has a tubercle on its neck in the tendon of the stapedius. 
The epihyal ( e.liy .) still show r s a point of cartilage in front of the facial nerve (VII.) ; 
it is fibrous below. 
Dissection of the Skull of a ripe embryo of Dasypus villosus (4§- inches long). — 
Fifth Stage (continued). 
This was the largest, but not the oldest, of my specimens of young and embryo 
Armadillos ; its centres of ossification were not, however, so much developed as in the 
specimen of Tatusia peba, which forms my fourth stage (Plate 5, figs. 5-8).* 
This is a much more massive kind of skull than that of a Tatusia, and the upper 
parts do not so completely overlie the lower; it is more generally outspread; evidently 
the genus Tatusia comes nearer to Myrmecophaga than does the genus Dasypus. 
The lower view (Plate 7, fig. 1) shows a very wide and very short snout, with the 
nostrils (e.n.) almost obliquely below. The base of the septum at this part is alate, 
and the valves of the openings ( n.v .) very large, and coiled upon themselves. The 
whole space occupied by these cartilaginous growths is transversely oval, and is neatly 
rimmed with bone. The nasals are only just hidden in this view by the cartilage 
(see fig. 2, n., al.n .); two-thirds of the rim is formed by the pre max diaries ( px .), 
which are twice as large as in Tatusia. Much of the facial plate of these bones can 
* These three kinds are all I have been able to obtain in their early stages; yet they show a most 
extraordinary amount of variation, and suggest to me, that it would be well worth while to work out 
the various stages of every species in the Family. 
MDCCCLX XXV. G 
