PROFESSOR HUXLEY ON THE OSTEOLOG-Y OF THE GENUS GLYPTODON. 55 
orbital part of the upper surface of the cranium is broad and nearly flat, smooth, and 
slightly concave at its posterior half, slightly convex, rough, and perforated by vascular 
foramina at its anterior half. The most prominent parts above the orbits are most 
rugose, and indicate a more intimate adhesion to the superincumbent osseous dermal 
helmet. The lachrymal foramen (r) is pierced immediately in front of the anterior 
border of the orbit. 
“ The difference in the development of the temporal muscles manifested by the Glypj- 
todon and Mylodon in the position of the ridges in the fossil cranium indicates a corre- 
sponding difference in the power of mastication and in the density of the alimentary 
substances habitually selected by each species ; the greater proportion of hard dentine 
in the teeth of the Glyptodon , and the greater number of the teeth, which appear to 
have been thirty-two, eight on each side of both jaws, coincide with the characters of 
the cranium, and support the inferences thence deducible.” 
It is necessary to make certain additions and qualifications to the above description. 
If we may be guided in the interpretation of the structure of the auditory region by 
the analogy of the existing Euphractus , the part which is there termed “ paroccipital” 
(Plate IV. fig. 5, h) includes the true mastoid; the “perforated depression for the 
digastric muscle ” (Plate IV. fig. 5,f) is the external auditory meatus; and that which 
is termed “ petromastoid below the digastric depression ” (Plate IV. fig. 5, g) is part of 
the tympanic element of the temporal bone. It would appear that, as in Euphractus, 
the tympanic bone sends a process outwards and backwards, the extremity of which 
comes into contact with the pars mastoidea, and so bounds the external auditory meatus 
externally and below ; while it leaves between itself, the proper tympanic bulla, and the 
pars mastoidea, an aperture which communicates with the external auditory meatus. 
The latter is remarkably small for so large an animal. The “ bulla,” into which it 
opened, is broken away ; but it is probable that a considerable part, if not the whole, 
of the rugose spaces supposed above to be for the insertion of “recti capitis antici,” 
mark the place where the thick inner walls of the bullae impinged upon the basioccipital. 
The fenestra rotunda is visible upon the under surface of the pars petrosa as an oval 
aperture 0T5 inch wide, the long axis of which is directed almost transversely to that 
of the skull. The fenestra ovalis , smaller, appears above the fenestra rotunda. The 
proper carotid canal probably traversed the anterior part of the internal wall of the bulla 
as in the Armadillos; the jugular vein most likely left the skull by a passage between 
the posterior and internal part of the bulla, the exoccipital, and the periotic. 
The large apertures perforating the roof of the cavity which is situated behind the 
articular facet for the lower jaw, do not communicate with the tympanic chamber. 
They are probably venous channels, and they communicate internally with the cavity of 
the skull. 
The articular facet for the lower jaw measures 1*8 inch along its greater, and 0-6 inch 
along its lesser diameter ; its edges are well defined, and it has a somewhat kidney-shape, 
the hilus of the kidney being turned downwards (Plate IV. figs. 4 & 5, e). The’general 
mdccclxv. K 
