In studying the _F. lanarius_, I have experienced most discouraging difficulties from the want of sufficient series of the Old World races, and from the unsatisfactory character of most descriptions and figures of them, besides being much perplexed by the confusion of their synonymy by different authors. In consequence of this, my diagnoses of the four races of which alone I have seen examples may be very unsatisfactory as regards the characters by which they may be most readily distinguished. Having seen the adult of only a single one of these four races, I am therefore compelled to base my differential characters upon the immature stages. In addition to the four races of _F. lanarius_ characterized above, there are several geographical forms belonging to the Old World, chiefly intertropical Asia and Africa. These are the var. _babylonicus_, Scl. and Irby, (Gray’s Hand List, I, p. 20, No. 173,) of Southeastern Europe and Western Asia; var. _barbarus_, L. (Gray’s Hand List, p. 20, No. 174), of Northern Africa; and var. _tanypterus_, Licht. (Gray’s Hand List, No. 175), of both the preceding regions, which Mr. Gurney writes me “is simply the intertropical race of _F. lanarius_, from which it only differs in being of a darker shade throughout.” The _F. saker_, Schleg. (Gray’s Hand List, No. 176), seems, to judge from the descriptions and figures which I have seen, to be also merely a form of the same species, but I have seen no specimens of it. Falco (Hierofalco) gyrfalco, LINN. Var. =candicans=, GMELIN. WHITE GERFALCON. _Accipiter falco freti hudsonis_, BRIS. Orn. I, 356, 1763. _A. gyrfalco_, BRISS. Orn. I, 370, pl. xxx, f. 2, 1763. _Falco rusticolus_, FABR. Faun. Grœn. p. 55, 1780.—LATH. Syn. Supp. I, 15, 1781. _F. candicans_, GMEL. Syst. Nat. p. 275, 1788.—DAUD. Tr. Orn. II, 101, 1800.—BENICK, Isis, 1824, 882.—SCHLEG. Krit. Ubers. p. 1, 1844.—BONAP. Rev. Zool. 1850, 484; Consp. Av. p. 33.—CASSIN, Proc. Ac. Nat. Sc. Phil. 1855, 278; Birds N. Am. 1858, 13.—STRICKL. Orn. Syn. I, 77, 1855.—BLASIUS, Cab. Jour. 1862, 43 (thinks all boreal ones same in Europe and America).—ELLIOT, Birds N. Am. pl. xii. _Hierofalco candicans_, CUV. Reg. An. ed. 1, I, 312, 1817; ed. 2, I, 323, 1829.—LESS. Man. Orn. I, 80, 1828; Tr. Orn. p. 97, pl. xvi, p. 2.—GRAY, Hand List I, 18, 1869. _Falco islandicus_, LATH. Ind. Orn. p. 32, 1790; Syn. I, 71, A, B; Gen. Hist. I, 72, A, 1821.—STEPH. Zool. XIII, pt. ii, p. 39, 1826.—GOULD, B. Eur. pl. xix.—AUD. Birds Am. 1831, pl. ccclxvi. _F. buteo_ β, LATH. Ind. Orn. p. 24, 1790; Gen. Hist. I, 80, A. _F. lagopus_, β, LATH. Ind. Orn. p. 19, 1790; Syn. Supp. I, 36; Gen. Hist. I, 68, A. _F. grœnlandicus_, DAUD. Tr. Orn. II, 157, 1800. _Hierofalco grœnlandicus_, BREHM. Voy. Deutsch, I, 16, 1831. _F. gyrfalco_, BONAP. List, p. 4, 1838.