136 Proceedings of the Biological Society of Washington. 



hand, Aphelocoma guerrerensis, a bird closely resembling Aphelo- 

 coma unicolor in coloration, although, by the way, a perfectly 

 distinct species, has the tail usually slightly longer than the 

 wing, in some cases about equal, and occasionally even shorter, 

 thus occupying an intermediate position, though certainly 

 more inclining toward the subgenus Aphelocoma. Were it not 

 for Aphelocoma guerrerensis, the case would be relatively sim- 

 ple, as all the other species fall well within the limits of one 

 group or the other, and, indeed, Sieberocitta could then well be 

 a distinct genus. 



This case of Sieberocitta is another good example of what we 

 consider the real difference between a genus and a subgenus, for 

 here two groups, which are well characterized by structural 

 differences, are connected by a species which is perfectly inter- 

 mediate. Did this species not exist, we would have two trench- 

 antly different groups which would naturally stand as distinct 

 genera. Many authors do not recognize subgenera, but such 

 divisions are often of considerable use in more clearly showing 

 the relative affinities of more or less closely allied species which 

 can not be separated generically. 



Although under existing circumstances we can not consider 

 Sieberocitta a distinct genus, the recognition of two subgenera, as 

 Mr. Swarth proposes, seems desirable. The forms of each are 

 as follows: 



Sieberocitta. 



Aphelocoma sieberii sieberii (Wagler). 

 Aphelocoma sieberii colimae Nelson. 

 Aphelocoma sieberii potosina Nelson. 

 Aphelocoma sieberii arizonae (Ridgway). 

 Aphelocoma sieberii wollweberi (Kaup). 

 Aphelocoma sieberii couchii (Baird). 

 Aphelocoma unicolor unicolor (Du Bus). 

 Aphelocoma unicolor coelestis Ridgway. 



Aphelocoma. 



Aphelocoma guerrerensis Nelson. 

 Aphelocoma cyanea (Vieillot). 

 Aphelocoma calif ornica calif ornica (Vigors). 

 Aphelocoma calif ornica immanis Grinnell. 



