44-O PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 



To this species I refer an incomplete skull in the collection of the 

 American Museum of Natural History (No. 9,507) which agrees well 

 with the type. In size and appearance this species is very near to the 

 preceding one, but the differences are characteristic and constant. The 

 upper incisors, which have plane and not convex faces, have slightly dif- 

 ferent proportions, the width and thickness being almost exactly equal. 

 The grinding teeth are distinctly wider than in the preceding species, 

 the two upper series have a stronger anterior convergence, and the exter- 

 nal pillar of m- is exceedingly small or absent. The hard palate is 

 wider behind, narrower in front than in P. erutus, but the posterior nares 

 are also wider and the palatines narrower. The rostrum has nearly the 

 same proportions as in the latter species ; the ascending ramus of the pre- 

 maxilla forms a narrow strip on the dorsal side of the rostrum, extending 

 a little behind the nasals ; the latter do not contract posteriorly and their 

 hinder portion is concave, not flat ; the forehead is much less convex and 

 inflated than in the larger species. 



MEASUREMENTS. 



Upper dentition, length 03 5 Zygomatic arch, length 03 1 



" " " pi-m-S. 015 Palate, length to base of rostrum 013 



Upper incisor, width 0023 " width at pi- 0025 



" " thickness 0023 " " " m^ 012 



Mi, length (i. e. antero-posterior diam- Rostrum, length 013 



eter) 003 " width at base 0085 



Ml, width (i. e. transverse diameter) . . . .004 " " " anterior end 007 



M^, length 0055 " dorso-ventral height 012 



" width 004 



PERIMYS SCALARIS Ameghino. 



(Plate LXVIII, Fig. 14.) 



Perimys scalaris Amegh. ; Rev. Argent, de Hist. Nat. ; T. I, 1891, p. 301. 



The type is a palate with all the grinding teeth in place ; it represents 

 an animal somewhat smaller than P. erutus, from which it is distinguished 

 by the prominent external pillar on all the upper molars. In the large 

 species of the genus the pillars of both upper and lower teeth are very 

 variable, but in P. scalaris and certain other small species they appear to 

 be quite constant. 



To the same species I refer a very young animal in the Princeton col- 

 lection (No. 15,063), in which p T has just been erupted and shows no signs 



