6 NORTH AMERICAN FAUNA. 



Among mammals the female is often less specialized tlian the male 

 and consequently bears more resemblance to the ancestral stock, thus 

 giving a clew to the line of descent when this can not be determined 

 from the male alone. In the present instance the females of norchora- 

 censisunA tropical is have small, smoothly rounded skulls without sagit- 

 tal crests and with narrow audital bulhe and inflated squamosals, as 

 in the cicognani series, while the males have large angular skulls with 

 well-developed sagittal crests, relatively broad audital bulla-, and flat 

 squamosals, as in the longicauda-frenatus series. The inference is that 

 the austral longicauda-frenatus series was derived from the boreal 

 cicognani stock, and that the differentiation took place in the South. 

 P. noveboracensis occupies middle ground geographically, and may have 

 become differentiated from cicognani under existing conditions in the 

 area it now inhabits; but P. tropicaMs, which inhabits tropical Mexico, 

 must either have originated from the cicognani stock when the latter 

 was driven southward by the cold of the Glacial epoch, or must have 

 accomplished a very remarkable migration. 



Turning now to the weasel of the tundras (P. arcticus], the female is 

 also found to resemble the cicognani type, indicating at least so far 

 as the American species go that the whole group (subgenus Ictu-) has 

 sprung from an ancestral type related to /*. cicognani. 



Probably cicognani itself is a strongly specialized type, although the 

 specialization took place a long time ago and seems to have been in 

 the direction of greater simplicity. The tendency has been toward a 

 narrowing of the skull as a whole and the obliteration of its promi- 

 nences and angles. The zygomata have been reduced and drawn in 

 close to the sides of the cranium, and the brain case has been nar- 

 rowed, elongated, and smoothly rounded off, as if to enable the head to 

 pass through small openings. The body as a whole has undergone 

 parallel modification, presenting the extreme degree of slenderuess 

 known among the mammalia. This type of weasel seems to have been 

 developed for the express purpose of preying upon field mice or voles, 

 its narrow skull and cylindrical body enabling it to enter and follow 

 their runways and subterranean galleries. The extreme development 

 of the type is presented in. P. rixosus and P. streatori, whose exceed- 

 ingly small size and almost serpentine form make it possible for them 

 to traverse the burrows of even the smaller mice. 



It is an interesting fact that the geographic range of the cicognani 

 group is almost coincident with that of the field mice of the subgenus 

 Microtus. Farther south, where these mice occur sparingly or not at 

 all, the cicognani series of weasels is replaced by the larger and more 

 powerful longicauda-frenatus series. Where the ranges of the two 

 overlap, as on the northern plains, the large weasel (P. longicaurfa) 

 preys chiefly on pocket gophers (Thomomys and Geomys] and ground 

 squirrels (Spermophilm franldini and 8. 13-lineatuft), while the smaller 

 species (cicognani and rixosus) prey chiefly on mice. 



