GLIRES OF THE SANTA CRUZ BEDS. 445 



Under the name of P. zonatus Ameghino has, I believe, included two 

 quite distinct species. The type of P. zonatus is remarkable for the ob- 

 liquity of its teeth, while the specimen here considered and upon which 

 the description given under the head of P. impacttis was founded, seems 

 to be very different. The description of P. impactus is as follows: "Size 

 as in P. onustus and P. zonatus ; distinguished by p T , i% and ^, which 

 have an internal perpendicular column behind. In P. onustus this column is 

 found only in p^ and n%; in P. zonatus it is present on all four molars" 

 ("94", 72). P. ameghinoivs a very large species, quite equalling P. impactus 

 in size ; it is characterized by p 4 , which hardly differs at all in shape from 

 the lower molars, being much less contracted on the inner side than in 

 the preceding species, and the valley opens more directly outward, less 

 anteriorly. In the only known specimen all four of the lower grinding 

 teeth have the internal column, but this may not be a constant character. 



MEASUREMENTS. 



Lower dentition, length pj-m^ 027 P^, width (i. e., transverse diameter) . . . .007 



P ?) length (i. e., antero-posterior M^, length 009 



diameter) 007 " width 006 



In the second section of the genus, corresponding to Sphiggomys, the 

 lower premolar is so turned as to have its major diameter almost coincident 

 with the long axis of the jaw, the valley opening nearly or quite directly 

 forward. Though the two groups of species are fairly well distinguished 

 by this character, transitions between them do occur. 



PERIMYS PUELLUS Ameghino. 



(Plate LXVIII, Fig. 4.) 



Sphiggomys puellus Amegh. ; Rev. Argent, de Hist. Nat. ; T. I, 1891, p. 



143- 



Perimys puelhis Amegh. ; fenum. Synopt. des Mamm. Foss. de Patagonie ; 



1894, p. 72. 



In the Princeton collection this species is represented by a fairly com- 

 plete skull, with mandible, associated with several cervical vertebrae and 

 bones, more or less complete, of the fore leg and foot. 



In size, this species does not greatly exceed P. erutus, but it differs 

 considerably from the latter in its proportions. The upper incisors are 

 not known ; those of the lower jaw are rather broad and have perfectly 



