ALLEN : MAMMALIA : MURID^. . 59 



well haired, especially on the apical half, and penicillate, bicolor, brown 

 above and whitish below. 



Adult (February to April). — The pelage is thinner, grayer and darker, 

 with less fulvous suffusion throughout, and the fulvous lateral line very 

 indistinct, evidently from fading. 



Youjig A(Yu//s {March and April). — Above dark gray, strongly varied 

 with blackish, and very faintly suffused with fulvous ; lower border of sides 

 tinged more or less strongly with fulvous, sometimes forming a poorly 

 defined lateral band ; ventral surface gray, sometimes without fulvous wash, 

 or varying from a faint buffy pectoral spot to a large strongly buff area 

 occupying most of the ventral surface. 



Measurements. — Six old males (from Basaltic Canons) measure: Total 

 length, 235 mm. (230-250); tail vertebrae, 115 (no- 125); hind foot, 

 30 (29-30). Four old females measure: Total length, 236 (225-242); 

 tail vertebrae, 115 (no- 119); hind foot, 30.6 (28-31). Young adults 

 range in total length from about 205 to 225, with a tail length of 100 to 

 1 10. Five adults from the coast (mouth of Rio Coy, all males except 

 one) measure: Total length, 240 (230-247); tail vertebrae, 114.5 (105- 

 116); hind foot, 30.6 (28-32). 



An average fully adult skull measures : Total length, 32 ; basal length, 

 27; zygomatic breadth, 16.5; width of braincase, 14; interorbital breadth, 

 4; palatal length, 14; palatal foramina, 7.5; diastema, 8; upper molar 

 series, 5. 



Represented by 24 specimens, of which about 17 are fully adult, the 

 rest being more or less immature. Five (3 adults and 2 young) were 

 taken on the upper Rio Chico in February ; 5, all adults, at the mouth 

 of the Rio Coy, Nov. 6-10; 6 (mostly immature) at Swan Lake in 

 March, and 9 at the Basaltic Canons in April. As noted above, they 

 represent three distinct phases of pelage, two of which are seasonal and 

 the other the adolescent stage. The Rio Coy series represents full 

 winter pelage, and the upper Rio Chico series the summer pelage ; the 

 Swan Lake and Basaltic Canons specimens are in the early fall pelage 

 (March and April), and differ strikingly from Rio Coy specimens, taken 

 in November. 



Mr. Waterhouse states (Voy. Beagle, /. c.) that there were three specie 

 mens of this species collected by Mr. Darwin ; the specimen in the British 

 Museum, designated as the type, is a rather young individual, and is in 



