IX DENTITION OF TYPOTHERIA 213 
account of the genus Protypothervwum. This animal was of 
about the size of a Hyrax, which indeed it resembles in several 
points of structure. The teeth have the primitive number of 
forty-four, and they are close set, leaving no diastema; the 
molars are rootless and grow persistently ; they are simple and 
Rodent-like in surface pattern. The shape of the lower jaw is 
like that of Hyraxz, being rounded in outline posteriorly ; there 
is no projecting angle as in the Rodents, and this remark 
applies to the Typotheria in general. The aspect of the Rodent 
lower jaw is characteristically different from that of Myrax and 
the forms under consideration. 
Some other characters of these early forms of Typotheria can 
be gathered from an inspection of other genera. In Jcochilus 
both hand and foot were five-toed, and, as in ancient Ungulates 
generally, the bones of the wrist and of the ankle are serially 
and not alternately arranged. Moreover, an os centrale is 
present in the carpus. Both thumb and big toe were opposable. 
The skull has a remarkably Rodent-like appearance, but the 
palate is not so narrowed as in these animals. 
In the more recent forms of Typotheria the dentition has 
become reduced. The canines are lost, and as the incisors are 
reduced also, to one on each side of the upper, and two on each 
side of the lower jaw, the likeness to a Rodent skull is increased. 
There is also evidence of a modification from the more primitive 
forms in the loss of one premolar or even more, in the alternating 
bones of the carpus, in the disappearance of the centrale, and in 
the loss of a toe upon the hind-foot. In these more recent 
forms the fibula articulates with the astragalus instead of with 
the caleaneum. Typotheria of these more recent forms may be 
illustrated by the typical genus Zypotherium. It has the re- 
duced dental formula I$ C°® Pm? M3; the molars are simple 
In pattern, and much like those of Zozodon. The upper incisors 
are powerful and curved, but are surrounded by a layer of enamel, 
which is not limited to the anterior face, as it practically is in 
rodents. ~The sacrum is composed of a large number of verte- 
brae—some seven—a state of affairs which recalls the Edentata. 
The shoulder blade is not Ungulate in form. It has a strong 
spine, with an acromion and a well-developed metacromion. The 
terminal phalanges are enlarged and hoof-like. 
In the genus Pachyrucos there are three premolars, otherwise 
