DEVELOPMENT OP THE SKULL IN THE MAMMALIA. 
37 
Thev, together, look like the opened shells of a Bivalve, their fore margin is rounded 
and the hind part somewhat cut away ; their sides are pinched in. 
Behind them come the pterygoids ( pg.); these bones have grown considerably on 
to the palate since the early stage (see Plate 2, fig. 6).* 
This addition to the hard palate is rare in Mammals, even ; and in this small 
degree is instructive, linking on these types to the Anteaters, where it takes place 
to the greatest degree. Their thickness also is worthy of note, for in another 
Neotropical Family of Edentata—the Sloths—the pterygoids are often curiously 
enlarged. 
Behind the short, thick, jugal (/.) the squamosals (sq.) are seen to be of great 
length, reaching as far to the back as the occipital condyles. 
They strongly clamp the auditory capsules over the tegmen, and opposite the 
manubrium mallei ( m.ml .) they show the pneumatic foramen, leading into their air- 
cell. A pair of superficial bones are seen at their inner border ; these are the “ annuli,” 
finished crescents of bone, now : they are more than semicircular, hollow within, and 
convex on their outside. Yet at present they do not ossify any part of the tube of the 
meatus externus, but when that segmented cartilaginous tube is removed, the mem- 
brana tympani ( m.ty .) is well exposed in this surface view. The endocranium is 
largely exposed behind the hard palate, and the bony centres of the posterior sphenoid 
and occipital arch are seen, as well as some parts of the auditory capsule. A very 
small part of the orbitosphenoid ( o.s .) is seen, where the optic nerve (II.) emerges. 
Behind these parts the alisphenoid is seen as one centre, right and left, confluent with 
the basal piece ( al.s ., b.o., read b.s.). The sphenoidal fissure and foramen ovale 
(V 1 ., 3 ., V 3 .) are not well seen. There is a considerable synchondrosis between the 
basisphenoid and the six-sided basioccipital (b.o.), and also between that bone and the 
exoccipitals ( e.o .) below, and between them and the large shield-shaped supraoccipital 
( s.o.), above. The emerging post-auditory nerves (IX., X., XII.) have their passages 
exposed, and the facial (VII.) is seen escaping from the stylo-mastoid foramen, behind 
the epihyal ( e.hy .). The tympanic annulus fails to cover the inner side of the 
cochlea (chi.), which is partly ossified. 
The peculiar form of the Armadillo’s skull is shown in the upper view (fig. 2). Three 
pairs of investing bones, the nasals, frontals, and parietals (n.,f, p.) cover nearly the 
whole of the top of the skull; the maxillaries, lacrymals, jugals, and squamosals 
(nix., sq.) are just seen at the sides; at the end, the convex top of the supra¬ 
occipital is seen, and in front, a small tract of the alse nasi ( al.n .). The nasals form, 
* I am now satisfied that my oldest (ripe) young of this kind of Armadillo belonged, like the smaller 
specimens, to the species liybrida. For my second stage (Plate 2, fig. 6) shows the pterygoids with a 
larger palatine tract than is seen in T. peba, one third longer (Plate 5, fig. 5). That skull is also longer 
and slenderer, whilst that of the older is like the skull of my second stage (Plate 2, fig. 6); both these 
are very broad in relation to their length. I feel sure that this ripe young is really T. liybrida, although 
given to me as T. peba. 
