﻿6 



NORTH AMERICAN FAUNA. 



[Xo. 11. 



Among mammals the female is often less specialized tlian the male 

 and conseqnently bears more resemblance to tlie ancestral stock, thus 

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

 from tlie male alone. In the present instance tlie females of iiovchora- 

 ccnsis and tropicalis have small, smoothly rounded skulls without sagit- 

 tal crests and with narrow audital biilhi3 and inflated squamosals, as 

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

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

 squamosals, as in the longicaufJa-f/enatus series. The inference is that 

 the austral longicauda-frenatus series was derived from the boreal 

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

 F. noreboracensis occupies middle ground geographically, and may have 

 become differentiated from cicognani \\\\({q^v existing conditions in the 

 area it now inhabits; but P. /ro/> 9, wliicli inliabits 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. arcticiis)^ 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 Ictis) has 

 simmg from an aacestral type related to P. cicognani. 



Probably cicognani itself is a strongly specialized t3^]>e, although the 

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

 the direction of greater simi)licity. 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 oft', as if to enable the head to 

 X)ass through small openings. The body as a whole has undergone 

 parallel modification, presenting the extreme degree of slenderness 

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

 developed for the express purpose of i)reying upon field mice or voles, 

 its narrow skull and cjdindrical 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 

 grouj) 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-frenaUis series. Where the ranges of the two 

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

 preys chiefly on pocket gophers [Thomomys and Geomys) and ground 

 squirrels {Sperniopliiln^^ franldini and aS'. 13-Iincatus), while the smaller 

 species {cicognani and rixosus) ])rej chiefly on mice. 



