T 9 O 3-] ^ llen^ New Rodents from Patagonia. 187 



mens the whole tip of the tail is black, including the under 

 surface. A few other specimens approach this condition. 



As shown by the measurements already given, the females 

 are much smaller than the males. 



The skull is very variable in respect to size and many de- 

 tails of structure, but especially in the size and form of the 

 interparietal. In one specimen it is almost obsolete, forming 

 a mere line less than a millimeter in antero-posterior extent 

 and 5 mm. in transverse extent. Generally it is subtriangu- 

 lar, with a transverse width of 5-7 mm. and an antero-pos- 

 terior length of 2-4 mm. It is sometimes divided medially 

 into two halves. On each side of the interparietal, and sep- 

 arated from it by the posterior extension of the parietals, is 

 an intercalated bone of variable size and of an irregularly 

 oval outline, each generally considerably larger in area than 

 the interparietal. 



Ctenomys robustus differs markedly in coloration from C. 

 magellanicus, but not very appreciably, so far as specimens 

 of the latter are available for comparison, in size or in cranial 

 characters. C. magellanicus is pale yellowish gray, or ash 

 gray with a fulvous tinge, while C. robustus is dark yellowish 

 brown. C. boliviensis is very much larger and very much 

 darker and redder, having "the general hue bright rufous 

 brown," and the upper surface of the nose, head, and nape 

 blackish. It appears to have no close relationship to any of 

 the other described species of Ctenomys. 



Ctenomys sericeus, sp. nov. 



Type, No. 84191, U. S. Nat. Mus., $ ad., Cordilleras, upper Rio 

 Chico de Santa Cruz, Patagonia, Feb. 5, 1897; O. A. Peterson. 



Type. Pelage short, soft, and glossy. General color above yel- 

 lowish gray strongly varied with black, the hairs being slaty plumbeous 

 for the basal three fourths, then banded narrowly with pale yellowish 

 brown, and tipped with black; flanks and ventral surface buff; sides 

 of nose yellowish brown ; top of nose and top of head like median dorsal 

 region, which is darker than the sides; ears very small, blackish; upper 

 surface of feet dingy gray with a slight yellowish cast; tail pale 

 yellowish, with a median dusky stripe along the apical half of the 

 upper surface. 



