26o 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
making the cranium relatively longer, though the brain-case proper, meas- 
ured from the postorbital constriction, is quite short. The whole skull is 
lower dorso-ventrally than in the Toxodonta, and the greatly shortened 
nasals, the much larger and obliquely placed anterior nares and the 
reduced premaxillaries, as well as the absence of large tusks, give a very 
different appearance to the entire facial region. The zygomatic arch, 
which is very broad dorso-ventrally, making it seem to be very heavy 
and massive in side view, does not rise so high or so steeply backward 
and conceals less of the cranial wall, while the dorsal border of the pos- 
terior portion descends quite strongly and unites with the occipital crest 
at a much lower level than in Nesodon or Adinotherium or the Santa 
Cruz typotheres, in all of which types the dorsal border of the zygomatic 
process at its origin is nearly on a level with the sagittal crest. The 
sagittal crest is quite long and very prominent, its development varying 
with age and perhaps with sex also. 
A very characteristic difference from the skull of the Toxodonta and 
Typotheria is the backward prolongation of the dorsal portion of the 
occiput, which extends well behind the plane of the condyles and, in 
side view, makes the occipital surface appear to be deeply concave, while 
in Nesodon and Adinotherium this surface is either almost vertical, or 
slopes forward and upward from the condyles. Though the structure of 
the auditory region is essentially the same as in Nesodon , the appearance 
of this region, when seen from the side, is very different. The very large 
tympanic bulla, with its great antero-posterior extension ; the long and 
freely projecting postglenoid process ; the small size, inferior position 
and non-projecting form of the external auditory meatus ; the very long, 
slender and antroverted paroccipital process, and, apparently, at least, the 
absence of any surface exposure of the mastoid portion of the periotic 
and of the mastoid process, are all highly characteristic features of this 
skull and clearly differentiate it from that of the Toxodonta and 
Typotheria. 
The upper view of the skull (PI. XXVIII, fig. id) has not the truncated 
triangular outline seen in the corresponding view of Nesodon and Adino- 
therium (cf. PI. XIV and PI. XX, fig. 3), but rather that of an elongate, 
somewhat narrow oval, with the maximum transverse width across the 
zygomatic arches a little behind the orbits, not at the glenoid cavity, as 
it is in the Santa Cruz Toxodonta. The backward projection of the 
