72 THE DESEADO FORMATION OF PATAGONIA 
Prosotherium triangulidens Ameghino 
. triangulidens Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 427. 
. robustum, Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 427. 
. quartum Amegh., 1901, Bol. Acad. Nac. Ciene. Cordoba, t. 16, p. 371. 
aaj sinoielas, 
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. 
vobustum, and having lower pm. 1 present. The latter 
character we found also characteristic of P. gargont, so 
only size remains and | do not consider less than 10% 
enough by itself to make a species. 
MEASUREMENTS 
Skull, length 110 mm. 
Upper dentition, length inc. I to m. 3 57 mm, 
Upper dentition, length pm. 1 to m. 3 35 mm. 
Upper dentition, incisor 1, width 8 mm. 
Upper dentition, molar 1, 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. 
