184 Mr. O. Thomas on 



Skull without marked supraorbital ridges or ledges. 



Dimensions of the type (measured in flesh) : — 



Head and body 72 mm. ; tail 72; hipd foot 165 ; ear 13*5. 



Skull: greatest length 21 ; condylo-incisive length 19'2; 

 zygomatic breadth 11'3 ; iuterorbital breadth 3"3 ; palatilar 

 length 8*8 ; palatal foramina 4*3 ; upper molar series 3*4. 



An older specimen measures : — Head and body 91 mm. ; 

 tail 85 ; hind foot 18; ear 15. Skull: greatest length 23*6 ; 

 condylo-incisive length 22'4. 



Hab. Province of Buenos Ayres ; type from La Plata City, 

 other specimens from Las Talas, Enseuada, and Bahia Blanca. 



Type. Young adult male. B.M. no. 6. 4. 6. 26. Original 

 number 148. Collected 8th June, 1896, and presented by 

 Oldfield Thomas. Fifteen specimens. 



This mouse is readily distinguishable from H. laucha by 

 its conspicuously longer tail, but is otherwise closely like 

 that species. It is the only member of the restricted genus 

 Hesperomys in which the tail is proportionally so long. 



Like the true laucha, this animal is not much of a burrower, 

 but lives about in the grass, and examples may be easily 

 obtained by kicking up loose heaps of hay, thistle-stalks, or 

 other rubbish, under which the mice take refuge. One 

 specimen I have marked as found under a heap of thistle- 

 stalks, in a round nest, the size of a tennis-ball, made of 

 thistle-down. Another was dug up, semi-torpid, in very cold 

 weather, from about 6 inches below the surface of the ground. 



There is also a Western form of the same species^ which 

 may be named 



Hesperomys murillus cordovensis, subsp. n. 



General characters as in true murillus of La Plata, but the 

 colour paler and less saturate, the back nearly matching 

 " drab " of Ridgway ; sides rather more buffy. Under sur- 

 face more heavily washed with buffy whitish, the slaty bases 

 of the hairs less evident. Colour of crown especially lighter 

 and more drabby than in true murillus. 



Skull appai'ently with a more inflated brain-case than in 

 murillus, but the number of fully adult specimens is not 

 sufficient to prove how far this is constant. 



Dimensions of type : — 



Head and body 85 mm. ; tail 74; hind foot 16; ear 14. 

 Another specimen measures : — Head and body 80 mm. ; 

 tail 84 ; hind foot 17 ; ear 15. 



Skull : greatest length 23*2 ; condylo-incisive length 2r5 ; 

 breadth of brain-case 10 8; upper molar series 3"3. 



