190 Mr. O. Thomas on 



Skull apparently quite as in puer. 



Dimensions of the type (measured in the flesh) : — 



Head and body 82 mm. ; tail 72; hind foot 20 ; ear 16. 



Skull : tip of nasals to back of interparietal 18 ; zygomatic 

 breadth 113 ; nasals 9; interorbital breadth 4*6; palatilar 

 length 10'2; palatal foramina 6 ; upper molar series 36 ; 

 breadth of to 1 TO. 



Hab. as above. 



Type. Adult male. B.M. no. 18. 1. 1. 38. Original 

 number 22. Collected 21st August, 1917. 



This Akodon seems alone related to A. puer, the remarkably 

 small size of the molars separating the two from any other 

 species known to me. In colour, as usual, the Jujuy form 

 is darker and less bright than that of the dry Bolivian 

 highlands. 



9. Hypsimys budini, gen. et sp. nn. 

 <$ . 38, 39, 40, 47, 48 ; ? . 43. Leon, Jujuy, 1500 m. 



Hypsimys, gen. n. 



External characters quite as in Akodon, but claws longer, 

 the anterior as long as or longer than the posterior. 



Skull in general form rather narrower than in Akodon, 

 but not of the excessive narrowness found in Deltamys. 

 Muzzle narrow. Supraorbital edges rounded ; slightly 

 squared posteriorly, but not ridged. Brain-case smooth, 

 without ridges. Interparietal present, but small. Zygo- 

 matic plate slanted in front, but not so narrowed as in 

 Oxymycterus and Microxus. Palatal foramina long. Bullae 

 not specially enlarged. 



Teeth. — Incisors slender, narrow, flat in front, fairly 

 orthodont in set, the index about 8.2°. Molars quite unique 

 in this group, highly hyp?odont, almost as much so as in 

 Chinchilhda, though of so different a type to that as to make 

 comparison difficult. They are just what Akodon teeth might 

 be expected to become if made very hypsodont, high, narrow, 

 with the vertical grooves extending far down towards the 

 roots, of simple sectional pattern, that of m 2 pandurate *, 

 that of m 1 similar but with three lobes, the front one circular, 

 and that of m 3 subtriangular. The roots are long and pecu- 

 liarly flattened laterally, and in consequence the alveolar 



* Ridgway, ' Nomenclature of Colour?,' 1st edition, 188G, pi. xiv. 

 fijr. 18. 



