This genus has no very near relatives among the American _Falconidæ_, nor, indeed, among those of other portions of the world. It is, perhaps, most closely related to the genus _Rupornis_, of South America, from which, however, it is very distinct. It is represented by a single species, the type of which, supposed to have come from California, still remains unique. The elongated legs, reaching considerably beyond the rather short tail, the close thigh-plumes, the long and extremely acute claws (somewhat like those of _Rostrhamus_), with the short, rounded, and very concave wing, are its most striking peculiarities. Besides these distinguishing features, the short, thick bill, very deep through the base, and the filamentous attachments to the shafts of the feathers of certain parts of the body, are also very characteristic. The latter feature may possibly be a mark of immaturity, but I have seen nothing like it in other _Raptores_, and it seems to be more analogous to the nuptial ornaments seen in the Cormorants (_Phalocrocoracidæ_). Species. =O. gruberi.= Wing, 10.10; tail, 6.50; culmen, about .80; tarsus, 2.70; middle toe, 1.45; posterior claw, 1.00, its digit .80. _Immature_ (?). Uniform grayish-umber, tinged with dull rufous on the neck; lining of the wing and tibiæ dull grayish-cinnamon. Primaries inclining to black, and showing just discernible, obscure hoary bars on their basal half. Tail brownish-gray, with a hoary cast nearer the shaft (not paler at the tip), and crossed with nine or ten narrow bars of dusky, these becoming hardly distinguishable basally and terminally. Inner webs of the primaries plain white anterior to their emargination. Head laterally and beneath obsoletely streaked with whitish. _Hab._ “California.” Onychotes gruberi, RIDGWAY. GRUBER’S HAWK. _Onychotes gruberi_, RIDGWAY, Pr. Ac. Nat. Sc. Phil. Dec. 1870, p. 149. [Illustration: _Onychotes gruberi._]