84 THE DESEADO FORMATION OF PATAGONIA 
Argyrohyrax acuticostatus Ameghino 
A. acuticostatus Amegh. 1901, Bol. Acad. Nac. Cienc. Cordoba, t. 16, p. 361. 
This species is described as differing from A. proavus 
in being somewhat smaller, but I find it about the same 
size. The specific character is in the teeth being more 
compressed, and in the anterior vertical margin of the 
upper molars being developed in the form of a very salient 
CrESE. 
Isoproedrium Ameghino 
Proedrium Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 623. 
Proedrium Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 431. 
Isoproedrium, Amegh, 1903, Anal. Soc. Cient. Argen., t. 56, p. 18. 
This genus is based on a toothless mandibular sym- 
physis of large size, on which the second incisor is indicated 
as larger than the first. We found one lower jaw, which, 
though imperfect, confirms the above as a genus and adds 
the following for generic characters; premolars with a 
shallow inflexion on the inner and outer sides, the molars 
with a shallow inner, and a narrow deep outer inflexion, 
heavy layer of cement on the premolars and molars, 
enamel reduced on the anterior internal, and the posterior 
internal corners of at least the premolars. 
Isoproedrium solitarium Ameghino 
loc. cit. above. 
I have assigned the specimen shown in fig. 51 to this 
species. Alveoli of the first and second incisors show inc. 
Fig. 51. Right mandible, dotted line indicated alveolus—natural size. 
2 considerably larger than inc. 1. About the third incisor 
and the canine my specimen shows nothing, being broken 
