244 Mr. O. Thomas on Mammals from 



Skull rather larger and more bowed than in Ct. osgoodi. 

 Nasals broad in front, ranging in eleven specimens from 9 to 

 10 mm. in greatest breadth, as compared with 7*8 in the 

 broadest of five specimens of osgoodi ; in correlation with this 

 the whole muzzle is broader ; posteriorly the nasals extend 

 almost or quite as far backwards as the fronto-premaxillary 

 processes, while in osgoodi the latter processes always 

 noticeably surpass the nasals. Height of crown above 

 alveolar margin about 17 mm. as compared with 15*5 in 

 osgoodi. Top of brain-case with the projecting piece of the 

 bullae which appears on the upper surface on each side of the 

 interparietal quite small (1-3 mm. in diameter) and distant 

 from the transverse occipital crest by at least its own diameter, 

 while in osgoodi this is larger (4-5 mm.) and is carried close 

 up against the front side of the crest. Interparietal longer 

 than broad, the converse being the case in osgoodi — commonly 

 divided by a median suture. 



Dimensions of the type (measured in skin) : — 



Head and body (apparently stretched) 260 mm. ; tail 98 ; 

 hind foot (s. u.) 40*5. 



Skull : condylo-basal length 57*5 ; greatest breadth S3* 7 ; 

 nasals 24x9'5; interorbital breadth 10 5; least breadth 

 across brain-case 18'2 ; posterior breadth on ridge behind 

 meatus 30*3; upper molar series (alveoli) 12*5. 



Hab. Valle del Lago Blanco, Cordillera region of Southern 

 Chubut, Patagonia (about 46° S., 71° W.). 



Tyye. Adult male. B.M. no. 3. 7. 9. 66. Collected 15th 

 August, 1899, by J. Koslowsky. Thirteen specimens ex- 

 amined. 



The species to which this Tuco-tuco is most nearly allied, 

 Ct. osgoodi, was obtained during the " Princeton University 

 Expedition to Patagonia " on the Upper Rio Chico, rather 

 more than two degrees south of the above locality. Two 

 topotypical specimens of it were presented to the British 

 Museum by the authorities of the United States National 

 Museum, and I have now been permitted the loan of three 

 further specimens for the purposes of the present description. 

 I have therefore had five examples of Ct. osgoodi for com- 

 parison with thirteen specimens of Ct.fodax, and find the 

 above distinctive cranial characters to be entirely constant. 



The relations of Ct. osgoodi with the Ct. magdlanicus of 

 still further south are still not very clear, as no satisfactory 

 series of the latter has yet been collected. But I suspect that 

 the two are exceedingly closely allied. 



