PROFESSOR OWEN ON THE MEGATHERIUM. 
577 
tion ; but a portion remaining on both sides shows that the malar ascended to the 
level of the antorbital foramen, Plate XXIII. fig. 2, r : it forms the lower and a great 
part of the hinder boundary of the orbit ; the latter by a triangular, slightly bent 
postorbital process (Plate XXI. fig. I, a), which almost touches the corresponding 
more slender process of the frontal (ib. 12 ). The ascending process (ib. c) is a long, 
narrow, unequal-sided triangle with an obtuse apex ; the descending process {d) is a 
longer and stronger one, extending, when the mouth is shut, outside and for three 
inches below the alveolar border of the lower jaw : its extremity is obtuse and re- 
curved. The fourth process (ib. h), which may be called the ‘zygomatic’ one, ex- 
tends beneath the end of the corresponding process of the temporal bone, but the 
obliteration of the suture in the present skull prevents a precise definition of its 
limits. The whole outer surface of the malar is slightly convex, moderately smooth, 
with a defined surface for muscular attachment near the back part of the base of 
the descending process. The inner surface shows, by its well-marked ridges and 
depressions, the vigorous action of the muscular fasciculi which derived their origin 
from that part. 
The orbit (Plate XXL fig. 1, 0 ), of proportionally small size, as in all large mam- 
malian quadrupeds, presents a long vertically oval form ; or rather, by the convex 
border of the malar (a), is reniforra. Its peripheral contour is almost completed by 
the descending postorbital process of the frontal (ib. 12 ) in tbe present skull ; anterior 
to which the prominent boundary is effaced by a broad smooth channel, where the 
orbital surface is more directly continued upon the facial surface of the maxillary : 
this part answers to the supraciliary notch in quadrupeds. The lacrymal bone being 
completely coaleseent, if not connate, with the maxillary, is recognisable only by the 
lacrymal foramen (ib. /), which is just within or behind the obtuse anterior border of 
the orbit. Admitting the essential presence of the lacrymal by this character, it then 
combines with the frontal, maxillary and malar bones, to form the contour of the 
orbit. Within this frame, the orbit, as already remarked, communicates extensively 
with the temporal fossa. 
The anterior aperture of the bony nasal canal (Plate XXIII. fig. 2, m, n, 22 ) is sub- 
circular, and is formed by the nasals, inaxillaries and premaxillaries ; the deep ver- 
tical sides being contributed wholly by the inaxillaries. 
The formation of the external bony aperture of the organ of hearing has already 
been described. 
Mandible . — The chief characteristic of the mandible or lower jaw is the near 
equality of its vertical to its horizontal or longitudinal extent, due to the height of 
the coronoid process, and more especially to the depth of the dentigerous part of the 
bone. The latter dimension relates to the interesting modification of the principle 
of maintenance of the efficiency of the masticating machinery, as contrasted with that 
in the great proboscidian quadrupeds with a similar diet to the Megatherium. 
The condyle of the jaw (Plate XXV. fig. 1, a) is transversely elliptical, 3 inches in 
