PROFESSOR OWEN ON THE MEGATHERIUM. 
573 
From the state of the sutures already alluded to, the limits of the superoccipital, 
parietal, and frontal bones cannot be defined. Above the arched superoccipital ridge 
(Plate XXII. fig. 2 , 3) there are two semielliptical rough depressions (a, a) for mus- 
cular attachments ; and, in advance of these, the upper surface of the cranium shows 
two other similar but shallower muscular impressions, b, h. The smooth surface of the 
parietal, gradually narrowed to an inch in width ( 7) between the temporal ridges {t), 
again as gradually expands into the frontal region (u), and it is perforated, a little 
anterior to the middle of the temporal fossa, by a submedian vascular (venous?) 
foramen (v), about half an inch in diameter. The temporal fossae (Plates XXL & 
XXII. 27, f, 12) are remarkable for their great antero-posterior extent, and for the 
encroachment upon them by the peculiar process (c) sent upward and backward 
from the malar bone, 26. They are partially defined anteriorly by the extension of a 
postorbital process (Plate XXL fig. 1, 12, Plate XXII. fig. 2, 12) downward to the malar 
bone (26); but, beneath and within this slender process, they communicate freely with 
the orbits, 0. The posterior boundary ridge, continued from that on the parietal bone, 
curves forward below and is continued into the sharp upper border of the zygoma, 
Plate XXI. fig. 1,27. The surface of the temporal fossae is grooved and perforated 
posteriorly by large vessels, and is everywhere strongly impressed by the attachments 
of muscular fasciculi. The base of the zygomatic process of the temporal bone has 
an extensive origin (Plate XXII. fig. 2 , t, 27), not less than 6 inches in antero-poste- 
rior extent ; its free portion, to where it joins the malar, being 3 inches in length. 
It is a strong trihedral bar of bone, rather concave on the upper and outer sides, and 
forming on the underside the glenoid articular cavity for the lower jaw. This cavity 
(Plate XXIV. g), of an oval form with the long axis transverse, measures 3 inches by 
2 f inches, and is half an inch in depth. Behind this cavity the base of the zygoma 
(Plate XXL fig. ,1,27) has coalesced w'ith the mastoid (s) and petrosal (le) elements of 
the temporal, which combine to form the meatus auditorius externus, e. This canal 
is subcircular, about 10 lines in diameter at the deeper part, where it is formed by 
the above elements, but doubtless wider at its outer part, where it was completed by 
the tympanic bone. This bone is wanting in the skulls of the Megatherium hitherto 
transmitted to England : the absence of any fractured surface upon the contour of 
the orifice of the auditory canal indicates, however, that the bone was a free element 
of the temporal in the Megatherium as in the Mylodon* and Glossotherium'f. 
The mastoid (Plate XXII. fig. 1,3) forms a rugged process, in depth or length not 
exceeding the pai occipital (4), but of greater breadth and thickness ; above it, exter- 
nally, and probably in the line of the primitive suture with the squamosal, is a venous 
foramen, Plate XXL fig. 1, 27. The petromastoid — probably the petrosal part in a 
greater degree — forms the hemispheric articular cavity (Plate XXIV. le) for the stylo- 
hyal (33), the anterior rugged wall of which cavity extends downwards farther than any 
* Description of the Skeleton of an Extinct Gigantic Sloth (Mylodon robustus, Owen), 4to, 1S42, p. 28. 
t Zoology of the Voyage of the Beagle, ‘ Fossil Mammalia,’ 4to, 1840, p. 59. pi. 16. fig. 4. 
4 F 2 
