ix DENTITION OF TYPOTHERIA 213 



account of the genus Protypotherimn. 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 

 Eodent-like in surface pattern. The shape of the lower jaw is 

 like that of Hyrax, 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 Eodent 

 lower jaw is characteristically different from that of Hyrax and 

 the forms under consideration. 



Some other characters of these early forms of Typotheria can 

 be gathered from an inspection of other genera. In Icocliilus 

 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 Eodent-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 Eodent 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 calcaiieum. Typotheria of these more recent forms may be 

 illustrated by the typical genus Typotherium. It has the re- 

 duced dental formula I ^ C % Pm f M f ; the molars are simple 

 in pattern, and much like those of Toxodon. 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 

 Eodents. 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 acrornion and a well-developed metacromion. The 

 terminal phalanges are enlarged and hoof-like. 



In the genus Pachyrucos there are three premolars, otherwise 



