2 Mr. R. Ridgway on the Genus Micrastur. 



M. gilvicoUis, Pelz. Orn. Voy. Novara { = M. rvficollis, plum- 

 beous phase, adult) . 



Fatco hucjinclien, Temm., = il/. ruficollis, Scl. & Salv., rufous 

 pliase (?), young. 



Falco xanthothorax, Temm.,=il/. ruficollis, rufous adult. 



Nisus concentricus, Less., is undeterminable ; and if the type 

 does not exist, the name may be thrown aside as entirely 

 worthless, though Pelzcln distinguished a very distinct species 

 by this name, which should be adopted for that bird"^. 



Micrastur guerilla, Cass.,^M. guerilla, Scl. & Salv. 



Micrastur zonothorax,CsLham^,==M. zonothorax, Scl. & Salv. 



My M. leucauchen (paper in Pr. Boston Soc.) is the young 

 of M. leucauchen of Scl. & Salv, ; and they, together, are young 

 and old jilumages of M. ruficollis. 



I was certainly wrong in referring so many of the recog- 

 nized forms to one species ; but this was partly owing to the 

 impossibility of making desirable comparisons. 



Before proceeding to give a diagnosis of the species I now 

 distinguish, it will be best to make a few remarks regarding 

 the stages of plumage assumed by them, and which, if clearly 

 borne in mind, will remove the greatest obstacle towards 

 understanding the species. In the first place, there is no 

 sexual difference in coloration, beyond what results from 

 irregular variations of an individual character ; in the next 

 place, in M. guerilla and M. ruficollis, there are two quite 

 different " phases " of plumage, corresponding in every jiar- 

 ticular to the grey and rufous plumages of certain Owls 

 (notably Scops, Glaucidium, and Syrnium aluco), and which 

 are most unquestionably entirely independent of sex, age, or 

 season. The grey phase may be taken as the normal one, 

 since the other is merely the evidence of a colour-variation, 



Pelzeln. It is distinctly stated to have the posterior lower parts ban-ed, 

 which the latter has not ; and there are other points which point to M. 

 leucauchen, Scl. & Salv. [At our request Mr. D. G. Elliot searched for 

 Vieillot's type of M. gilvicoUis in the Paris Museum, but failed to find it. 

 We now think, with Mr. Ridgway, that the name M. concentricus had 

 best be used for the Guiana bird. — Ed. J 



* [Lesson's type does exist, and is doubtless the bird here called 31. 

 £oncentricus, and also by v. Pelzeln. — Ed.] 



