PLAGIARTHRUS 63 



by the external and internal folds which almost meet in 

 the median line. On the outside, each tooth is coated with 

 a layer of cement. When better known it may prove that 

 this genus, so specialized in the character of the teeth, 

 does not belong in this family. 



Plagiarthrus clivus Ameghino 



P. clivus Amcgh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 436. 



This species is represented by a single specimen from 

 the Chico del Chubut, west of Puerto Visser, which pre- 

 serves pm. 3 and 4 and the molars. 

 The characters of this, the type 

 species, are those of the genus. Fig 2g ^ lower premolars ? and 

 The total length of the five teeth is 4 and molars '-3-^tuiai size. 

 36 mm., and fig. 28 shows in natural size the various in- 

 dividual teeth. 



Hegetotheriidae Ameghino 



This family includes a large variety of forms from the 

 formations from the Deseado up to the Mt. Hermosa, but 

 all agree in having the first upper and the first two lower 

 incisors enlarged into strong gnawing teeth; in the reduc- 

 tion or absence of in. 3, the canine, and premolar I of the 

 upper and lower dentitions; in having the external face of 

 the upper molars not inflexed ; in lower molar 3 being three- 

 lobed; and in the bulla being inflated and hollow. There 

 are in the family two series of forms, at least, the one lead- 

 ing to the running Hegetotherium, the other to the hopping 

 Pachyrukhos, and the very little known form Phanophilus 

 which may fit into one of the other series when better 

 known. 



In the Deseado the following genera are assigned to 

 the family. 



Prohegetotherium, like Hegetotherium, except that the 

 last premolar and the molars have a vertical furrow near 

 the external anterior margin. 



