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PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
The brain-case is very narrow, making the temporal openings very large, 
giving the skull an almost reptilian appearance and allowing the pterygoid 
processes of the alisphenoids to be plainly visible. The forehead broadens 
greatly and abruptly in front of the postorbital constriction, and the face 
narrows very gradually forward from the orbits, though widening again 
very slightly at the muzzle. Another very characteristic feature of this 
skull is the manner in which the facial region narrows dorsally, the narrow 
and convex nasals forming the whole upper surface in front of the orbits. 
The solid and massive premaxillaries project for a considerable distance 
in front of the nasals, but much less than in Toxodon , in which the nasals 
are greatly reduced in length. 
The base-view (PI. XV) of course corresponds closely with the upper 
view in the form of the outline, but shows somewhat more distinctly that 
the widening of the muzzle, which is very slight, and, indeed, hardly per- 
ceptible, begins at the anterior end of the maxillaries, at the insertion of 
the canines. This widening is very much greater and more abrupt in Toxo- 
don :, and even in the Santa Cruz genus Adinotherium it is much more 
decided than in Nesodon . This view also shows clearly the peculiar form 
of the posterior nares, the prominent lateral projection of the pterygoid 
processes of the alisphenoids and the great breadth of the auditory region, 
on each side, external to the paroccipital process. The mammillate tym- 
panic bullae appear unduly small, as their principal diameter is dorso- 
ventral. The broken stylo-hyals are also visible, attached to the anterior 
side of the bullae, a highly exceptional arrangement. 
The basi-occipital is quite long and stout, though narrowing between 
the tympanic bullae and expanding again behind them, and bears a moder- 
ately prominent median keel ; it takes no part in the formation of the 
condyles, which are confined to the exoccipitals. The latter are very low 
and wide, but form relatively little of the occipital surface, except at the 
base, and having no crest. The foramen magnum is low and very broad, 
of transversly oval shape. The condyles also are low and broad, quite 
closely approximated ventrally, very widely separated dorsally. Though 
they are almost sessile, they project quite strongly backward, owing to the 
slope of the exoccipitals, and are fully visible when the skull is seen in 
side-view. Each exoccipital bears a very long paroccipital process which 
is heavy and trihedral proximally, becoming much more slender and 
rounded distally. The supra-occipital is very large and forms the greater 
