GLIRES OF THE SANTA CRUZ BEDS. 389 



available, and only the fuller knowledge of the pre-Santacruzian rodents 

 can lead to definite results. 



HYSTRICOMORPHA. 



OCTODONTID^.. 



CAPROMYIN^. 



NEOREOMYS Ameghino. 



(Plates LXIV, Figs. 6-6"; LXV, Figs, i-n ; LXX.) 



Neoreomys Amegh. ; Enumeracion systematica, etc. ; 1887, p. 10. 

 Pseudoneoreomys Amegh. ; Rev. Argent, de Hist. Nat; T. I, 1891, p. 300. 



This genus includes the most abundant of the Santa Cruz rodents, and 

 its species are among the largest in size. The teeth are quite similar to 

 those of the recent Myocastor (Myopotamus] though somewhat simpler in 

 pattern and without cement in the valleys. On the other hand, the skull 

 is much more like that of Capromys, with certain distinct resemblances to 

 that of Agouti (Coslogenys}. 



Dentition. The incisors are broad and heavy, with nearly plane or very 

 slightly convex anterior faces and sometimes with a very faintly marked 

 groove near the external border ; they differ in proportions from those of 

 .Myocastor. The grinding teeth vary greatly in appearance according to 

 age and amount of wear. The single premolar in each jaw is erupted 

 very late, not until the last molar is in use. In the unworn state p A 

 (Plate LXV, fig. i) is divided into three laminae by oblique furrows, which 

 are directed inward and backward across the masticating surface ; at the 

 end of the anterior valley, near the antero-external angle of the crown, is 

 an isolated enamel pillar. In the moderately worn state of this tooth the 

 crown is columnar, undivided by sulci, with oblique enamel lakes, indi- 

 cating the course of the original enamel folds or valleys, of which the 

 posterior is the smallest. Ameghino has distinguished two genera in 

 accordance with the number of these lakes, Pseudoneoreomys with three, 

 and Neoreomys with a fourth, produced by an internal fold. Probably 

 this difference is merely a matter of abrasion and is determined by the 

 mode in which the original three folds are subdivided by wear. 



