2io FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. X. 



able fulvous on the sides of the belly and likewise on the throat; on the 

 other the under parts are practically pure white. The upper parts are 

 quite as in P. brevicai4dus. I see no objection to treating this form, if 

 it be a valid one, as a subspecies of brevicaudus, for whatever local 

 variations there may be, it is beyond speculation that all are relatively 

 recent derivatives of a common stock. The fact of such slight differ- 

 entiation in animals from localities so distant from each other is also 

 an indication of general continuity of distribution and probable sub- 

 specific relationship. 



Measurements of the two adults are: Head and body 259, 243; 

 hind foot (dry) 50, 47.5; greatest length of skull 57.8, 60.2. 



Ctenomys opimus Wagner. TUCO-TUCO. 



One specimen, Oruro. 



This is somewhat immature and noticeably paler than a single topo- 

 type of C. opimus with which it has been compared. Thomas* has 

 referred specimens from Oruro to this species and has specially men- 

 tioned a wide color variation among individuals from one locality, so 

 our specimen probably has no extraordinary peculiarities. It is to be 

 remembered, however, that rodents of fossorial subterranean habits 

 are usually subject to much local differentiation, and when our know- 

 ledge of the tuco-tucos is on a par with that of their North American 

 analogues, the Geomyidae, we will doubtless recognize a much larger 

 number of forms than at present. 



Gavia musteloides boliviensis Waterhouse. BOLIVIAN CAVY. 



Twenty-four specimens, Parotani. 



A series of this size all taken at one time and place affords a valuable 

 opportunity to observe variation, especially since such series thus far 

 have seldom been preserved. In both color and cranial features con- 

 siderable variation appears; rather more, generally speaking, than is 

 found among cricetine and murine rodents. The color of the upper 

 parts is relatively uniform throughout the series, the general tone being 

 slightly darker or lighter according as the buffy annulations of the 

 black-tipped hairs are narrower or broader. The broad, basal color of 

 the hairs of the upper parts varies narrowly between shades of mouse 

 gray and smoke gray always being darker mid-dorsally than laterally. 

 The color of the under parts ranges from pale buff or creamy to the 

 roots of the hairs (except on the throat) to creamy whitish with a dark 

 grayish undercolor extending throughout except on the inner sides of 

 the legs where the hairs are pale and self-colored. This latter type is 



*Ann. & Mag. Nat. Hist., (7), IX, p. 227, 1902. 



