232 PROCEEDINGS OF THE ACADEMY OF [1894. 



in other members of the subfamily (m ' with 

 three salient and two re-entrant loops on each 

 side ; ni ^ and -^ each with three salient and 

 two re-entrant loops on outer side and two 

 ,,. , salient and one re-entrant loop on inner side); 



liiiio). Left upper the re-entrant enamel folds from both sides 

 "|"l:»i'^- stopping on or very near median line of teeth, 



and directed jnward at nearly right angles to long axis of teeth, 

 instead of obliquely backward as in the other genera ; m - and '^ 

 subequal in size ; m ' slightly larger ; anterior loop of m ' pro- 

 jecting on inner side as far as middle and posterior loops, and falling 

 short of plane of other two on outer side (reversing the usual con- 

 dition); zygomatic root of maxillary reaching back to posterior part 

 of m ^ (anterior part in other genera). 



Tretomys atavus Aiiiesliiuo. (Fig. 2.) 



Trctonivs atiwiis Anie.uhiiio, ^Mnniilei-ds Fosiles Repiib. Argentina, ISSit, Hi), 

 120, pi. 4,'tigs. 10 unci U\a. 



Based on fossil fragment of maxillary bone containing molar 

 series, and upi)er incisor, from near city of ( -ordoba, Argentine 

 Republic. 



Specific characters. — Most of the characters have been given 

 above, in the generic diagnosis. The molars are implanted squarely 

 one in front of the other, the longitudinal axis of each tooth being 

 the same as that of the series collectively. In the other genera the 

 ui)per molars are implanted obliquely, the axis of m ^ and •> sloping 

 outward as well as backward from the axis of the series as a whole. 

 In Tretomys the posterior loop on the inner side and the anterior 

 loop on the outer side of ra ^ and •' are more largely developed 

 than in the other genera, the result being that on each side all 

 of the salient loops of the series end nearly on the same plane. 

 Ameghino's measurements of the molar series are: Series, 5 mm.; 

 m ', 2 mm.; m '^,1.6 mm.; m ■^ 1.4 mm. The upper incisor is 

 1 mm. broad and its face is very convex. 



Genus HODOMYS' nob. 

 (PI. IX, Figs. 1-4, 7, S, and Fig. H, a, />, e, (I, in text.) 



Type Ncolo))ia allcni Merriam, Troc. Biol. Soc. Wasli., VII. .Sept. 189;2, Il)7-l(i(» 

 (Type from Manzauillo, Mexico). 



9 Hodomys, from orfof road, and fivg mouse, in allusion to the road-making 

 liahit of both species. 



