GLIRES OF THE SANTA CRUZ BEDS. 443 



and plane ; posteriorly its alveolus forms a marked protuberance on the 

 inner side of the jaw, extending nearly to m T . On p T , the internal pillar 

 is very faintly marked ; indeed, in this individual, the pillar is well devel- 

 oped only on m , though it is fairly distinct on IT% also. The horizontal 

 ramus of the mandible is shallow and slender and the symphysial region 

 is comparatively narrow. 



In the Ameghino collection is a very fine skull with mandible, which is 

 referred to P. onustus, but the incisors are so much heavier than in the indi- 

 vidual here described that it probably belongs to a different and perhaps 



undescribed species. 



MEASUREMENTS. 



Lower dentition, length 051 P^, width (i. e., transverse diameter). . . .0055 



" " pj-1% 025 M 7 , length 008 



Lower incisor, width 0032 " width 006 



" " thickness 0033 Mandible, depth at PJ 0115 



PI length (*'. e., antero-posterior diam- " " " notch in front of p^ .0075 



eter) 006 



PERIMYS IMPACTUS Ameghino. 



(Plate LXVIII, Figs, i, la, 2, 2a, 9, ga.) 



Perimys impactus Amegh. ; Enum. Synopt. des Mamm. Foss. de Pata- 

 gonie; 1894, p. 72. 



This species is not only larger than P. onustus, but is also of relatively 

 more robust and massive character. The incisors are considerably heavier 

 than in the latter and have more convex faces ; the upper premolar is 

 large, projecting outside of the other teeth and giving a sigmoid curva- 

 ture to the external line of the dental series ; the upper molars normally 

 have the antero-external pillar. The lower premolar is much contracted 

 on the inner side and an internal pillar seems to be constantly present, 

 but varies much in size ; in one individual (Plate LXVII, fig. ga], for 

 example, it is not observable in the grinding surface, though plainly vis- 

 ible from the inner side ; some individuals have a second pillar near the 

 antero-external border. On the lower molars also the pillar is variable ; 

 it is constantly present in m 7 and has not been observed on m T , while on 

 m^ it may be large and conspicuous, small, or entirely absent. 



The skull (PL LXVIII, figs, i, 10) is very large; it bears considerable 

 resemblance to that of Viscaccia, but is actually much longer and rela- 

 tively narrower. The occiput is higher and not so broad and the periotic 



