2 Mr. R. Ridgway on the Genus Micrastur. 
M. gilvicollis, Pelz. Orn. Voy. Novara ( = M. ruficollis, plum¬ 
beous phase, adult). 
Falco leucauchen, Temm., = M. ruficollis , Scl. & Salv., rufous 
phase (?), young. 
Falco sc ant hot hor ax, Temm .,=M. ruficollis, rufous adult. 
Nisus concentricus, Less.,, is undeterminable; and if tbe type 
does not exist, the name may be thrown aside as entirely 
worthless, though Pelzeln distinguished a very distinct species 
by this name, which should be adopted for that bird* *. 
Micrastur guerilla, Cass.,=M. guerilla, Scl. & Salv. 
Micrastur zonothorax, Cabanis,=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 plumages 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 93 of plumage, corresponding in every par¬ 
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 barred, 
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. gilvicollis in the Paris Museum, hut failed to find it. 
We now think, with Mr. Ridgway, that the name M. concentricus had 
best he used for the Guiana bird.— Ed.] 
* [Lesson’s type does exist, and is doubtless the bird here called M. 
concentricus, and also by v. Pelzeln. —Ed.] 
