442 PATAGONIAN EXPEDITIONS: PALAEONTOLOGY. 



m r ~ s in place. It is evident from the shape of the empty alveolus that 

 p T was much compressed and very narrow in the inner side, yet the val- 

 ley apparently opened laterally, not anteriorly ; on all of the lower molars 

 the internal pillar is strong. 



To the same species I refer a palate, with all the grinding teeth in place, 

 in the Princeton collection (No. 15,227), which is from a slightly larger ani- 

 mal than the type. This specimen is remarkable for the large size of p A , 

 which projects externally beyond the line of the molars and gives a de- 

 cided curvature to the outer line of the dental series, a curvature which is 

 more marked than in the larger species ; mi and 1 have antero-external 

 pillars. 



In the table the dimensions of the upper jaw and teeth are from No. 

 15,227, the length of the lower grinding series is from the type in the 

 Ameghino collection. 



MEASUREMENTS. 



Upper dentition, length pA-m& 0215 M-S, length 0075 



PA, length (i. e., antero-posterior diam- " width 005 



eter) 005 Palate, width at pA 004 



P-*., width (i. e., transverse diameter) . . .0065 " " " m^ 022 



Ml, length 004 Lower dentition, length p-^-m-g 020 



" width 005 



Ameghino has described three large species of Perimys, distinguishing 

 them by the development of the pillars, but, as already mentioned, these 

 pillars are very variable in the large species, and in the same jaw a given 

 tooth on one side may have a prominent pillar and on the other side be 

 without a trace of it. Nevertheless, the three species may be distinguished 

 upon other grounds and are probably valid ; at all events, it would be 

 premature to unite them. 



PERIMYS ONUSTUS Ameghino. 



(Plate LXVIII, Figs. 3, 3".) 



Perimys onustus Amegh. ; Enumeracion sistem., etc.; 1887, p. 12. 



This name may be applied to the somewhat smaller individuals, in 

 which the incisors are narrow and the mandible relatively slender. The 

 specimen which best agrees with the type is a right mandibular ramus 

 (No. 15,230) especially characterized by its slenderness, which, in turn, 

 is due to the relatively low-crowned teeth. The lower incisor is narrow 



