100 On small Mammals from the Delta of the Parana. 



specially narrowed behind. Mesopterygoid fossa broad, twice 

 as wide at its anterior end as the base of the narrow ptery- 

 goids on each side of it; in most members of the Akodont 

 group the base of eacli pterygoid is as broad as the fossa 

 between the two. Bullte small, as in Akodon. 



Incisors rather thrown forward, their angle with the molar 

 surface 80°-S3°. Molars light and slender, their structure 

 as already referred to. 



Dimensions of the type (measured in flesh) : — 



Head and body 89 mm. ; tail 85 ; hind foot 22; ear 13. 



Skull: greatest length 25*6; condylo-incisive length 24; 

 zygomatic breadth 11*7 ; nasals 9'2 ; interorbital breadth 4:'4 ; 

 breadth of brain-case 10"8 ; palatal foramina 5'7 ; breadth of 

 mesopterygoid opening 1*5 ; upper molar series 4. 



Type. Adult male. B.M. no. 17. 6. 1. 35, Original 

 number 2830. Collected 1st February, 1917. 



This little blackish mouse is a most interesting addition to 

 the fauna of Eastern Argentina, being so very different from 

 any other animal known there. Like Oxymycterus tomen- 

 tosus, similarly dark-coloured, it is no doubt peculiar to the 

 marshy and water-logged region of the Parana delta, and has 

 on this account escaped discovery until Mr. Kemp's visit to 

 the islands. 



'' From neglected orchard " ; ^' from the jungle pajanal,'' 



7, Oxymycterui platensis, Thos. 

 S . 2827, 2844 ; ? . 2820. 



8. Myocastor coypus bonari'ensis, Geoff. 

 Two skulls. 



9. Cavia pampariim, Thos. 

 Four skulls. 



10. Blastocerus dichotomus, 111. 



? . 2850. 



" Presented by Mr. F. E. Welch. — Snared by a quintero." 

 —R.K. 



11. Lutreolina crassi'caudata, Desm. 

 (?. 2811,2816. 



