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PATAGONIAN EXPEDITIONS ’ PALAEONTOLOGY. 
he has pointed out, the polyphyletic origin of the mammals from reptiles 
and is thus made extremely improbable from the standpoint of present 
knowledge. So far as I am aware, his theoretical conceptions have not 
found any support even among those who accept his homologies of the 
toxodont skull. 
Each of the three suborders has its peculiarities in the structure of the 
auditory region. In the Santa Cruz Typotheria the tympanic bulla has a 
long, tubular, auditory meatus, the opening of which has a relatively low 
position and is directed backward rather than outward. The pars Serrialis 
of the squamosal and even the root of the zygomatic process have a very 
inflated appearance externally and are either hollow, as in Pachyvuklios , 
or, much more commonly, filled with cancelli. The mastoid (post- 
tympanicus of Roth) has no process, the distal end being closely applied 
to the paroccipital ; it is ankylosed with the tympanic and usually with 
the squamosal, in which case it appears to form the post-tympanic pro- 
cess. The hyoid arch is loosely attached to the skull and is inserted in 
a deep pit on the outer side of the bulla, external to the paroccipital 
process. 
The comparison of the Typotheria and Toxodonta, as regards the audi- 
tory region, is by no means clear. What would seem to represent the 
mastoid (the protuberancia petrosa ) of other ungulates is conspicuous in 
the Toxodonta as a thin plate which ends in a well defined process ; it 
is ankylosed with the bulla and may, in fact, be an outgrowth of the 
tympanic. If it is really the mastoid, it occupies a highly exceptional 
position, being separated from the exoccipital by another element, which 
appears to be the post-tympanic process (or pars Serrialis ) of the squa- 
mosal, and is almost invariably continuous with the squamous portion, 
though I have detected traces of a sutural connection with the latter. 
There is no visible tube leading to the external ear-opening, the passage 
being entirely concealed by the structures just described. The opening 
has a very elevated position at the postero-external angle of the zygo- 
matic arch, much as in the pig and, indeed, the whole appearance of the 
occipital surface is suggestively like that seen in the latter animal. The 
tympanic bulla is, as in both of the other suborders, large, completely 
ossified and hollow, free from cancellous bone ; in shape it is mammillate, 
with the principal diameter dorso-ventral. A most exceptional character, 
at least in the Santa Cruz genera of the suborder, is the position of the 
