The American Lanner Falcon is so very closely related to the Lanners of Europe and Asia (var. _lanarius_ and var. _jugger_) that it is very difficult to indicate the differences which separate them. The two Old World forms above named are more unlike each other than they are from the two American races; the var. _jugger_ differing from _mexicanus_ apparently only in larger size; and the var. _lanarius_, more like _polyagrus_ than it is like either _jugger_ or _mexicanus_, differs from _polyagrus_ mainly in the greater amount of white on the plumage, this imparting a lighter aspect to the pileum, and causing a greater development of the light spots on the outer webs of the primaries and rectrices. [Illustration: _Falco polyagrus._] The var. _polyagrus_, compared with var. _lanarius_, is much darker, having, at all ages, the crown uniformly brown, with darker streaks, instead of having these streaks upon a white ground. The “mustache” is more distinct in the American bird, while in the European the bands on the tail are much more distinct, and the spots forming them are on the outer webs, as well as on the inner, instead of on the latter alone; the dark bars between the light spots are in the American bird much narrower and more numerous, and in the young the light ones come to the edge of the web, instead of being enclosed within the dark color. Two very young birds (i.e. in first perfect plumage) appear almost identical until closely examined, the chief differences being a lighter tint to the crown in the European, and heavier dark stripes on the breast, besides the peculiar character of the tail-spots, which are always distinctive. In shades of color, there is not the slightest difference. I have seen no specimen of any of the Old World forms in the plumage corresponding to that transversely barred above, described here as the adult, though figures of the adult _lanarius_ indicate a very similar plumage. The series of the latter race at my command is unfortunately limited to a very few immature specimens. One marked “ad.” (56,051, Hungary; Schlüter Coll.) measures as follows: Wing, 14.50; tail, 8.00; culmen, .83; tarsus, 1.90; middle toe, 1.80. Its colors are as described in the synopsis (p. 1429) for the young bird.