40 PATAGONIAN EXPEDITIONS: ZOOLOGY. 



Ctenomys robnstus 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 cranial characters. C. magellanicus is pale yel- 

 lowish gray, or ash gray with a fulvous tinge, while C. robnstus is dark 

 yellowish brown. C. bolivieiisis 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 described species of Ctenomys. 



It was met with only in the alluvial river valley of the upper Rio 

 Chico, at the eastern base of the Andes. Mr. Peterson says of this spe- 

 cies : "Very abundant, especially along streams. The ground they occupy 

 is all undermined with their passages, which cross one another in all 

 directions. They are seldom seen on the surface and are difficult to trap." 

 (MSS. notes.) Most of the specimens of this species appear to have been 

 taken in Mayer Basin, at the edge of the Cordilleras, since in writing of 

 this region Mr. Hatcher refers incidentally to Mr. Peterson's obtaining 

 here a splendid collection of rodents, "including a fine series of a much 

 larger species of Cteiiojiiys than any we had seen in the plains region " 

 (Narrative, p. 138). 



Ctexomvs sericeus Allen. 



(Plate Vlir, Figs. I and 2, Skull.) 



Ctenomys sericeus Allen, Bull. Am. Mus. Nat. Hist., XIX, 187. May 9, 

 1903. Upper Rio Chico de Santa Cruz, near the Cordilleras, Pata- 

 gonia. 

 Ctenomys maget/anica Hatcher, Princeton University Exped. Patagonia, 

 I, Narrative, 1903, 124. Not of Bennett. 



Pelage short, soft, silky and lustrous. General color above yellowish 

 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 yel- 

 lowish brown ; top of nose and top of head like median dorsal region, 

 w'hich 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. 



In some specimens there is a tendency to a well-marked darker median 

 dorsal band, extending from the nose to the base of the tail. Several of 

 the specimens are a little darker than the type above described. The tail 



