CHAPTER V 



TYPOTHERrA 



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. 



