62 THE DESEADO FORMATION OF PATAGONIA 



by enamel, which is opposite the internal inflexion, and 

 in a young individual, is presumably connected with the 

 fold. In the same way, the last three lower premolars and 

 the lower molars each have an internal pit, adjacent to 

 the external inflexion. With advanced age all the teeth 

 show closed roots, another primitive feature. In spite 

 of the closed roots, the full dentition, and the enamel on 

 the incisor; and on account of the deep inflexions and the 

 isolated pits, I consider this genus a specialized side line, 

 retaining many primitive features, and expect to find the 



Fig. 27. Archaeohyrax patagonicus, after Ameghino natural size. 



ancestor of the typotheres in some one of the related Casa- 

 mayor genera. 



Ameghino described three species, A . patagonicus, which 

 we have figured, and which has a length of 84 mm. from 

 inc. I to m. 3 in both the upper and lower dentitions; A. 

 propheticus, of the same size, but with the dental series 

 closed; and A. concentricus of larger size, the three lower 

 molars having a length of 38 mm. 



Plagiarthrus Ameghino 



Plagiarthrus Amcgh., 1897, Bol. Inst. Gcog. Argen., t. 18, p. 436. 



This genus is known only by the lower premolars and 

 molars, which are permanently growing teeth, composed 

 of two subcylindrical cylinders almost entirely separated 



