Tlic TTca.sc/s of Eastern North America. 3 



than in any of the others, while in P. lonf/icauda the sexes are 

 more nearly alike. In adult examples of P. norehoraceiuis the 

 male averages about eighty millimeters more in total length than 

 the female, while in louf/icnuda, which is a larger animal, the dif- 

 ference is about sixty millimeters. This sexual difference in size 

 must always be kept in mind in trying to identify weasels, since 

 it may give rise to a great deal of trouble, especially in skins 

 made up by inexperienced collectors, who are nnable to deter- 

 jiiine the sex of their specimens. 



Apart from the difference in size, the sexes of most weasels are 

 alike, noveboracensis being the only one to show other sexual char- 

 acters (in this species there is a well marked sexual difference in 

 the skull apart from size). 



Variation ivith age. — Weasels var}^ much in size with age, con- 

 tinuing to grow for at least a year, and probabl}^ after the age 

 can be told by the skull. Very young weasels in the first sum- 

 mer have the tails rather short, the hair of the tail very short and 

 appressed, and the tail tapering off to a point without any de- 

 cided pencil. In this condition the tail has a very different look 

 from that of tlie adult. The color of the under parts is often 

 more yellow or Iniffy in the young than in the adults, but on 

 the whole the young and old do not differ much in external ap- 

 pearance. 



The skull of course varies greatly with age. In young skulls 

 the brain case always looks very large, round, and deep. By 

 actual measurement, however, it is about as in the adult, and as 

 the animal increases in age the rest of the skull grows U|) to it. 

 The whole rostral portion of the skull is slender and small in the 

 young and gradually becomes broader and heavier as the animal 

 grows older. This change takes place slowly and seems to con- 

 tinue over a long period. Very old examples of any species, but 

 especially of P. richardsoni and P. r. cicognani, show very broad, 

 heavy rostrums. This is often the surest mark of great age, which 

 a worn and broken condition of the teeth by no means always 

 indicates. All the species of the longica.uda group and the male 

 of P. noveboracensis develop strong sagittal crests with age, while 

 the members of the ricliardsoni group and the female of P. nove- 

 boracensis keep quite smooth and show only a slight indication 

 of a sagittal crest. The sutures all close very early, and the skull 

 has the appearance of age long before it has attained full size. 



Indindiud variation. — The range of individual variation in color 

 is slight and unimportant. In some forms, noticeably in P, rich- 



