GLIRES OF THE SANTA CRUZ BEDS. 405 



In molar pattern, the genera of the subfamily form a continuous series 

 of increasing complexity, which may be tabulated thus : 



A. Lower grinding teeth with two internal folds. 



a. Incisors with convex faces Stichomys. 



b. Incisors with plane faces Adelphomys. 



B. Pj and m-j- with three internal folds, m^ and y with one Gyrignophus. 



C. Py, m T and 7 with three internal folds, my with one Spaniomys. 



D. My y with three internal folds, p^- unknown Graphimys. 



STICHOMYS Ameghino. 



(Plate LXV, Figs. 16-20.) 



Stichomys Amegh. ; Enumeracion sistemat, etc.; 1887, p. 10. 



This genus comprises small rodents, with rooted molars and narrow 

 incisors, of which the thickness exceeds the breadth and of which, except 

 in one species, the anterior faces are strongly convex. The size of the 

 grinding teeth varies considerably with age, diminishing and becoming 

 more uniform as abrasion advances. The molar pattern is simpler than 

 in the other genera of the subfamily, Adelphomys excepted. In the upper 

 jaw the teeth have each one deep and narrow enamel fold from the inner side 

 and nearly opposite to this a corresponding external fold, the two being 

 connected in unworn specimens ; in front of the principal external fold is 

 another and somewhat shallower fold from the outer side, and in quite 

 unabraded teeth is probably a third, posterior external fold. I have seen 

 no specimen, however, in which this hindermost valley is not already 

 converted into a lake. As abrasion increases, the anterior fold first be- 

 comes isolated and finally even the principal internal and external folds 

 are cut off, the peripheral enamel becoming continuous and enclosing 

 four lakes. The lower molars are somewhat simpler in pattern, having 

 one external and two internal folds and lacking the posterior fold of the 

 upper teeth. I have seen no example of the milk teeth ; dp4 was prob- 

 ably shed very early, as the premolar is always the most worn of the 

 permanent teeth. 



The skull is very incompletely known and in none of the individuals is 

 the significant occipital and periotic region preserved. The upper contour 

 of the skull is slightly arched in the antero-posterior direction, very much 

 as in Loncheres. Seen from above, the general appearance of the skull 

 has considerable resemblance to that of the recent genera, Echimys and 



