GLIRES OF THE SANTA CRUZ BEDS. 413 



GRAPHIMYS PROVECTUS Ameghino. 



(Plate LXV, Fig. 26.) 



Graphimys provectus Amegh. ; Rev. Argent, de Hist. Nat.; T. I, 1891, 



p. 300. 



This is a very small animal, about equalling Spaniomys modestus in size. 

 Length m T g- = 7 mm. 



ERETHIZONTID^E. 



ERETHIZONTIN^E. 



STEIROMYS Ameghino. 



(Plate LXVI, Figs. 1-9.) 



Steiromys Amegh. ; Enumeracion sistem., etc.; 1887, p. 9. 



It is interesting to note that this genus is more like the recent North 

 American Erethizon than it is like the modern South American genera of 

 the same family. Indeed, it is difficult to frame a generic diagnosis 

 which will satisfactorily separate it from Erethizon, though there are many 

 differences of detail and the number of these differences would doubtless 

 be increased, if more complete material were accessible. 



The incisors are very heavy, conspicuously more so than in the recent 

 genera, and may either be of equal width and thickness, or the latter di- 

 mension may be slightly greater. In the upper jaw the incisors have 

 feebly convex faces and are very much curved antero-posteriorly, ending 

 in front of p- ; in the mandible the faces are plane, with very narrow and 

 shallow groove near the external border, which is raised and pillar-like ; 

 these teeth are much longer than in the modern genera of the family and 

 extend behind m into the ascending ramus, which gives quite a different 

 appearance to this region of the jaw. 



The grinding teeth have the same pattern as in Erethizon and Coen- 

 dou, with low crowns, covered with thick enamel, and long roots ; in the 

 masticating surface the valleys are hardly so deep as in the former, but 

 the anterior valley is complicated by a more or less interrupted ridge, 

 which is not present in the recent genera. The two rows of upper teeth 

 are nearly parallel, and though slightly more convergent anteriorly than 

 in Coendou, are much less so than in Erethizon. I have seen no example 

 of the milk-teeth, though it is evident from the comparative degree of 



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