72 THE DESEADO FORMATION OF PATAGONIA 



Prosotherium triangulidens Ameghino 



P. triangulidens Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 427. 



P. robustum, Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 427. 



P. quartum Amegh., 1901, Bol. Acad. Nac. Cienc. Cordoba, t. 16, p. 371. 



This species is similar to P. garzoni except for size, the 

 forms running about 12% larger, and being heavier built. 

 In this same line the upper and lower molars are relatively 

 wider and heavier. The top of the skull also is wider. 

 I have drawn carefully the skull and dentition so that the 

 detail can be seen from the figures. Beside triangulidens, 

 Ameghino described P. robustum, which, as far as I can 

 see, differs only in being about 5% larger, which is well 

 within individual variation, so I have considered it as a 

 synonym. The same is the case with P. quartum, which 

 Ameghino distinguishes as being about the size of P. 

 robustum, and having lower pm. I present. The latter 

 character we found also characteristic of P. garzoni, so 

 only size remains and I do not consider less than 10% 

 enough by itself to make a species. 



MEASUREMENTS 



Skull, length no mm. 



Upper dentition, length inc. I to m. 3 57 mm. 



Upper dentition, length pm. I to m. 3 35 mm. 



Upper dentition, incisor I, width 8 mm. 



Upper dentition, molar i, width 4-! mm. 



Six specimens from Chico del Chubut 

 Propachyrucos Ameghino 



Propachyrucos Amegh., 1897, Bol. Inst. Geog. Argen. t. 18, p. 425. 



The genus is based on lower jaws, in which the characters 

 of the premolars, the molars and the first two incisors, 

 resemble those of Pachyrukhos; but in this genus the third 

 incisor, the canine, and the first premolar are retained 

 and but little reduced. Ameghino has described two 

 species, P. smithwoodwardi, and P. aequilatus. 



