444 PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 



is less extensively exposed upon the occipital surface, though much more 

 so than in the small species of Perimys. The sagittal crest and temporal 

 ridges are more prominent and the parietals on each side of the crest are 

 more concave than in Viscaccia; the tympanics are more largely inflated 

 and more spheroidal in shape, and are not suturally connected with the 

 pterygoids. The forehead is more convex than in the small species of 

 the genus, owing to the large development of the frontal sinuses, and the 

 postorbital processes are long and prominent. The posterior nares are 

 relatively broad. 



The mandible differs from that of P. onustus in the much greater depth 

 and thickness of the horizontal ramus and in the greater height of the 

 alveolar border above the edentulous region. In one specimen (No. 

 15,697, PI. LXVIII, figs. 2, 2") the jaw is unusually thick and massive and 

 the masseteric crest is more prominent than in the other individuals ; to 

 some extent, this great thickness may be abnormal or even pathological. 



MEASUREMENTS. No. No. No. 



Type. 15,881. 15,053- 15,697- 



Upper dentition, length p^-m^ 030 .028 



P, length (z. e., antero-posterior diameter) 007 



" width (i. e., transverse diameter) 0095 



M&, length 009 



" width 006 



Lower dentition, length p^-m^ 028 .027 



?- ( , length 0055 .007 .008 



" width 007 .0065 .0075 



Mj, length 0055 .0085 



" width 008 .007 



Occiput, height .030 



" width at base .040 



Skull, width in interorbital region .043 



Zygomatic arch, length .052 



Palate, width at pA .005 



" " " ml .018 



Mandible, depth at p T .018 .017 



" " " notch in front of p^ .on .013 



" thickness at p ? .013 .0145 



PERIMYS AMEGHINOI sp. nov. 



(Plate LXVIII, Fig. 10.) 



Perimys zonatus Amegh., in part; Enum. Synopt. des Mamm. Foss. de 

 Patagonie, 1894, p. 72. 



