GLIRES OF THE SANTA CRUZ BEDS. 433 



ACAREMYS MINUTISSIMUS Ameghino. 



( Plate LX VII, Figs. 11, 11".) 



Acaremys minutissimus Amegh. ; Enumeracion sistem., etc.; 1887, p. 9. 



This is the smallest rodent yet discovered in the Santa Cruz beds, being 

 decidedly smaller than Sciamys tenuissimus, from which it may be dis- 

 tinguished not only by the convex faces of the lower incisors, but also by 

 the form of the mandible, the ascending ramus rising more anteriorly and 

 externally, leaving a wider space between itself and the teeth. 



A specimen in the Princeton collection is even smaller than the type, 

 in which the lower grinding series measures 5.5 mm. in length and the 



depth of the jaw below p T is 4 mm. 



* 



MEASUREMENTS. 



Lower dentition, length oio Lower incisor, width 0005 



" " " p T -m 3 005 " " thickness ooi 



" " " Pj-m^ 004 Mandible, depth below p^ 0033 



ACAREMYS TRICARINATUS Ameghino. 

 Acaremys tricar^nat^ls Amegh. ; Enum. Synopt. des Mamm. Foss. de 



Patagonie; 1894, p. 68. 



Known only from the type specimen. In all of the preceding species 



of Acaremys p T is of the same pattern as the molars, but in the present 



.species it is more complex, having two external enamel folds, instead of 



only one, thus producing three external crests. Size as in A. murinus. 



I am not entirely satisfied that this species is not founded upon a young 



individual of the last-named species with dp T still in use. 



CHINCHILLIDA1. 

 PERIMYS Ameghino. 



(Plate LXVIII, Figs. 1-15.) 



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

 Sphiggomys Amegh. ; Ibid. 



This genus is abundantly represented in the Santa Cruz beds and in- 

 cludes a surprising number of species, which, in size, range from very 

 small animals to the largest rodents yet found in the formation. 



