GLIRES OF THE SANTA CRUZ BEDS. 439 



PERIMYS ERUTUS Ameghino. 



(Plate LXVIII, Figs. 5, 13.) 



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



The typical and most abundant species and of quite small size, though 

 by no means the smallest of the genus. The upper incisors are slender 

 and have moderately convex faces ; the grinding teeth are relatively nar- 

 row and in nv 1 the anterior lamina projects so as to form an external 

 pillar, the prominence of which varies considerably ; the other molars have 

 no such pillar. The two dental series converge anteriorly, but less 

 strongly than in certain other species. In the mandible p T is but moder- 

 ately compressed on the inner side and the valley opens almost laterally ; 

 only in m-g is an internal pillar formed by the projection of the posterior 

 lamina beyond the anterior one. 



The occiput is low and wide, the breadth considerably exceeding the 

 dorso-ventral height ; the periotic seems to be excluded from the occipital 

 surface and to be exposed only in a narrow fossa between the squamosal 

 and exoccipital. The tympanic element of the auditory bulla is very largely 

 inflated and closely appressed to the basioccipital, leaving but a nar- 

 row slit for the foramen lacerum posterius ; the posterior nares are narrow 

 and the palatines are relatively broad. The rostrum is slender and elon- 

 gate, somewhat constricted at the base and slightly enlarged at the max- 



illo-premaxillary suture. 



MEASUREMENTS. 



Upper dentition, length 03 5 Occiput width at base '. 021 



" " " p-t-mi 014 Tympanic bulla, antero-posterior diam- 



Upper incisor, width 002 eter Oil 



'.' " thickness 0023 Zygomatic arch, length 028 



Ml, length (i. e., antero-posterior diam- Palate, length to base of rostrum 015 



eter) 003 " width at pA 003 



Ml, width (i. e., transverse diameter). . .0035 " " "ml oio 



M, length 005 Rostrum, length 017 



" width 0033 " width at base 008 



Skull, length 059 " " " anterior end 007 



Occiput, height 014 



PERIMYS PROCERUS Ameghino. 



(Plate LXVIII, Fig. 7.) 



Perimys procerus Amegh. ; Contrib. al Conoc. de los Mam. Fos. de la 

 Repub. Argent; 1889, p. 172. 



