I 14 PATAGONIAN EXPEDITIONS'. PALAEONTOLOGY. 
ancia petrosa of Roth). This position of the mastoid between the two 
processes of the squamosal is an altogether exceptional one, the normal 
place, when it appears on the surface of the skull, being between the ex- 
occipital and the squamosal. Inward, the process is extended to a contact 
with the tympanic bulla, with which it may coossify. 
The hyoid apparatus possesses the peculiarity (unique among mammals, 
so far as is now known) of being attached to the anterior , instead of the 
posterior, end of the tympanic bulla and in the adult skull the stylohyal 
is firmly ankylosed in that position. In the Santa Cruz Typotheria the 
hyoid is loosely connected with the skull and is very rarely found in posi- 
tion, but its point of attachment is different from that of the Toxodonta, 
though likewise very exceptional. In this group the junction of the par- 
occipital process with the tympanic leaves no room for the hyoid in its 
usual position and it is displaced to the external side of the bulla, near 
the posterior end, where a cylindrical fossa may be seen in some of the 
genera. 
The neck is short and thick and the trunk is very long, with long and 
deep thorax. The development of the neural spines is quite different in 
the various genera. In the comparatively small Adinotherium the spines 
of the back and loins have their tips in nearly the same horizontal plane, 
making the profile an almost straight line, while in Nesodon the spines of 
the anterior thoracic vertebrae are greatly elongated and form a decided 
hump at the shoulders, an arrangement which is exaggerated in Toxodon . 
The thoracic vertebrae have the pedicles of the neural arch perforated by 
large foramina for the passage of the spinal nerves. The sacrum is long, 
broad and depressed. Caudal vertebrae are not yet known in the Santa Cruz 
representatives of the suborder, but from the character of the sacrum it 
may be inferred that the tail, as in Toxodon , was heavy and of moderate 
length, in contrast to the long and slender tail of most of the contempo- 
rary Typotheria. 
The limbs are in all cases relatively short and heavy and the feet sur- 
prisingly small in proportion to the size of the animal ; the limb-bones 
differ greatly in size and massiveness in the various genera of the sub- 
order, but agree quite closely in structure among all the forms in which 
these elements are known. One striking difference between the earlier 
and the later genera is in the shape of the scapula, which in the Santa 
Cruz animals is relatively much broader than in the Pampean types and 
