CHAPTER V 
TYPOTHERIA 
In the Deseado beds this group of running and hopping 
animals is well represented, making about 14% of the 
Amherst collection, and varying in size from a little larger 
. than a rat to larger than a sheep. 
The group all have the front teeth modified into cropping 
or gnawing types, which grow permanently from persistent 
pulps; and the back teeth also growing through the whole 
or a large part of life, and also rootless, the crowns being 
variously infolded to make grinding surfaces. The skull 
is flattened above, and abruptly truncated behind; the 
cranium being large and swollen, the facial portion broad 
above and excavated on the sides. The orbits are centrally 
located, of considerable size, and unbounded behind. The 
tympanic bulla is swollen and may be hollow or filled with 
cancellous tissue. This cavity of the tympanic is con- 
tinued above and expands in the upper part of the squa- 
mosum, making a swollen capsule on either side of the back 
of the cranium. The openings of the auditory meatus 
are well back and in a tubular growth of the periotic which 
is directed back, and upward in an entirely characteristic 
manner. The strong paroccipital processes project far 
below the base of the carnium. The concave palate is 
wide and carried well back behind the teeth ending in 
two strong pterygoid processes. The mandible is deep, 
especially the back portion; has a slender coronoid process, 
and a small rounded articular condyle which would seem 
to indicate a forward and backward motion of the jaws. 
On account of the agreement with these general features, 
I have placed among the Typotheria the forms which Ame- 
ghino classified as Hyracoidea. 
