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. 1 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 Amegh., 1897, Bol. Inst. Geog. 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 
