The species of this genus are exceedingly numerous, about fifty-seven being the number of nominal “species” recognized at the present date. Among so many species, there is, of course, a great range of variation in the details of form, so that many generic and subgeneric names have been proposed and adopted to cover the several groups of species which agree in certain peculiarities of external structure. That too many genera and subgenera have been recognized is my final conclusion, after critically examining and comparing forty of the fifty-seven species of Gray’s catalogue (Hand List of Birds, I, 1869, pp. 29–35). The variation of almost every character ranges between great extremes; but when all the species are compared, it is found that, taking each character separately, they do not all correspond, and cross and re-cross each other in the series in such a manner that it is almost impossible to arrange the species into well-defined groups. From this genus I exclude _Lophospiza_, Kaup (type, _L. trivirgatus_); _Asturina_, Vieill. (type, _A. nitida_); _Rupornis_, Kaup (type, _R. magnirostris_); _Buteola_, Dubus (= _Buteo_, type, _B. brachyura_, Vieill.); included by Gray under _Astur_, as subgenera, and _Tachyspiza_, Kaup (type, _T. soloensis_); and _Scelospiza_, Kaup (type, _S. francesii_); which are given by Gray as subgenera of _Micronisus_, Gray (type, _Accipiter gabar_), the species of the typical subgenus of which, as arranged in Gray’s Hand List, I refer to _Nisus_. All these excluded names I consider as representing distinct genera. The species of this genus are noted for their very predatory disposition, exceeding the Falcons in their daring, and in the quickness of their assault upon their prey, which consists chiefly of small birds. SUBGENUS NISUS, CUVIER. _Accipiter_, BRISSON, 1760.[81] _Nisus_, CUVIER, 1799. (Type, _Falco nisus_, LINN.; _A. fringillarius_ (RAY), KAUP.) _Jerax_, LEACH, 1816. (Same type.) _Cooperastur_, BONAP. 1854. (Type, _Accipiter cooperi_, BONAP.) _Hieraspiza_, 1844, _Jeraspiza_, 1851, and _Teraspiza_, 1867, KAUP. (Type, _Falco tinus_, LATH.) _Hieracospiza_, AGASS. (Same type.) _Urospiza_, 1845, _Urospizia_, 1848, and _Uraspiza_, 1867, KAUP. (Type, _Sparvius cirrhocephalus_, VIEILL.) _Erythrospiza_, KAUP, 1867. (Type, _A. trinotatus_ (TEMM.?)) _Micronisus_, GRAY, 1840. (Type, _Falco gabar_, DAUD.) _Nisastur_, BLAS. 1844. (Same type.) [Illustration: =10759=, ♂. NAT. SIZE _Nisus fuscus._] [Illustration: =26588=, ♀. ½ =26588=, ♀. ½ _Nisus cooperi._]