26 PATAGONIAN EXPEDITIONS: ZOOLOGY. 



Adults vary individually in the amount of yellowish suffusion above, 

 some being quite strongly yellowish, while others are dark gray with very 

 little tinge of yellow. 



Measitrenients. — Eight adult and semiadult males measure as follows : 

 Total length, 217.5 mm. (210-230); hind foot, 50 (49-52). Eight adult and 

 semiadult females: Total length, 218.8 (210-230); hind foot, 48.5 (47- 

 52). The skulls show that few of the specimens of which the measure- 

 ments are here given are full grown. No. 84180, a male, is the only very 

 old individual, with the skull heavily ossified, in the series. This speci- 

 men has a total length of 230, and the length of the hind foot is 52. 

 The skull of this specimen measures : Total length, 54 ; zygomatic breadth, 

 32; interorbital breadth, 10.5; greatest width of brain case, 25 ; mastoid 

 breadth, 22; length of nasals, 19; palatal length, 24; palatal foramina, 

 7.4; diastema, 13; upper molar series, 13.5 ; lower jaw, length from inner 

 base of incisors to posterior border of condyle, 36 ; do., to end of angular 

 process, 42 ; height at condyle, 14 ; lower molar series, 13.5. 



The next oldest skull is that of a female, No. 84182, which has the 

 same external measurements as the male, namely, total length, 230, hind 

 foot, 52. The skull, howevef, is considerably smaller, measuring as fol- 

 lows : Total length, 47 ; zygomatic breadth, 29 ; least interorbital breadth, 

 10; greatest width of brain case, 21 ; mastoid breadth, 15; palatal length, 

 20 ; palatal foramina, 5.3 ; diastema, 10.8 ; upper molar series, 1 1.2 ; lower 

 jaw, length from inner base of incisors to posterior border of condyle, 31 ; 

 do., to end* of angular process, 35; height at condyle, 12; lower molar 

 series, 11.3. 



Represented by 29 specimens, of which 20 were collected by Mr. Col- 

 burn near Swan Lake and the Basaltic Canons, in March and April, and 

 the remaining 9 by Mr. Peterson, on the upper Rio Chico de Santa Cruz, 

 in the valley close to the river, near the Cordilleras, during the month of 

 February. Thus the pelage of only one season is represented. 



This animal evidently continues to increase in size for a long period. 

 Of the 29 specimens in the present collection only one or two, or at most 

 three, appear to have reached fully adult conditions, and these do not show 

 indications of old age. In this respect they resemble the Geomyidae among 

 North American Rodents, and the DidelpJiis group among Marsupials. 



Charles Darwin's notes on the habits and distribution of this animal, as 

 published by Waterhouse in " Zoology of the Voyage of the Beagle," are 



