476 LAND MAMMALS IN THE WESTERN HEMISPHERE 



These animals had mostly hypsodont teeth, forming roots in 

 old age, and the teeth were in closed series, but there was no 

 tusk-like enlargement of the incisors. In the later genera, 

 those of the Patagonian stage ("fNotohippus, ]Argyrohippus), 

 the crowns of the grinding teeth had a thick covering of cement, 

 and those of the lower jaw had some resemblance, though not 

 at all a close one, to the teeth of horses. The skull also had a 

 certain suggestion of likeness to the horses and Dr. Ameghino 

 was persuaded that these animals were ancestors of the horses. 

 The family went back to the Astraponotus stage, but can be 

 traced no farther. 



Suborder fTYPOTHERiA. jTypotheres 



This suborder was composed of much smaller animals than 

 the fToxodonta and contained no large forms ; some, indeed, 

 were exceedingly small, no larger than rabbits. It was much 

 the most diversified of the suborders, as is made evident by the 

 table of families and genera. Two of these families, the 

 fTypotheriidae and the jHegetotheriidae, continued into the 

 older Pleistocene. Of the former there was the genus first 

 named and described, ]Typotherium, which has given its name 

 to the family and suborder, and the species of which were much 

 the largest of the entire group, almost equalling a large pig in 

 size. At the first glance this genus might easily be mistaken 

 for a large rodent, and indeed it has actually been referred to 

 that order, but the resemblance was a purely superficial one 

 and involved no relationship. 



In '\Typotherium the teeth were considerably reduced in 

 number, the formula being : i ^, c^, p f , m f , X 2 = 24, The 

 first incisor in each jaw was a broad, scalpriform, persist- 

 ently growing tooth, which much resembled the corresponding 

 tooth in the rodents, but was not, as it is in the latter, worn to 

 a sharp chisel-edge by attrition, but was abruptly truncated. 

 There was a second similar, but much smaller, tooth in the lower 

 jaw ; the other incisors and all the canines had been lost and 



