64 NELSON 



some specimens the nape patch is only slightly marked or entirely re- 

 placed by the general whitish color; it is absent in a few specimens 

 from near Tehuantepec, and becomes more and more obsolete to the 

 south. Our large series, representing both summerandwinterspecimens, 

 shows great variation in color of underparts. Midsummer specimens 

 with few exceptions vary from pale to deep reddish buff ; the under 

 side of tail along median line is usually paler (buffy gray, or yellow- 

 ish rufous), and the ear patches are absent or very small. Specimens 

 taken in winter, and up to close of dry season in May or June (with 

 half a dozen exceptions), are rich rufous below. 



A young female from Huilotepec (May 8) is dingy gray below with 

 the buff restricted to axillary and inguinal regions, the middle of the 

 back rather darker and more rusty and the sides grayer, in greater con- 

 trast with the back than usual. 



An adult female from Tonala in summer pelage differs from all the 

 others in having the entire back strongly suffused with light rusty red, 

 thinly grizzled with grayish white ; underparts dingy yellowish white 

 faintly and minutely grizzled with black ; lower surface of tail grizzled 

 dull orange buffy and black, edged with white. Specimens on which 

 rufous ringed hairs predominate on the back are suffused with rufous. 

 In the series from Puerto Angel north of Tehuantepec City the back 

 is much paler or more grayish white, the nuchal patches darker, and 

 underparts deeper rufous than in typical socialise but in one case the 

 back is uniform grayish white with no sign of a nuchal patch. In the 

 series from the vicinity of Tonala south of Tehuantepec the nuchal 

 patch is less strongly marked and usually absent; the subterminal ru- 

 fous on the back is much more conspicuous, often showing through as 

 a strong yellowish rusty suffusion grizzled with white and black ; and 

 the underparts vary from pale buffy whitish to dull ferruginous. 



Measurements . — Average of five adults from type locality : total 

 length 524.4; tail vertebrae 271.4; hind foot 66.6. 



Cranial characters. — Premolars \. Skull very similar to that of 

 S. aureogaster. Five adult specimens from the type locality aver- 

 age : basal length 51. 8; palatal length 26.9; interorbital breadth 18. i ; 

 zygomatic breadth 33.9; length of upper molar series 11.3. 



General notes. — This species was described from an immature 

 specimen taken by the botanical collector Karwinski, who reported 

 that it frequented the forest on the southern coast of the Isthmus of 

 Tehuantepec in droves. The idea of its gregarious habits, suggesting 

 Wagner's name, must have come from seeing it during the mating 

 season, when a number are often found together, as in the case of other 



