The species of this subgenus are generally of small size and slender form; but with a graceful and apparently delicate structure they combine remarkable strength and unsurpassed daring. They differ from the species of _Astur_ mainly in less robust organization. The species are very numerous, and most plentiful within the tropical regions. The Old World possesses about thirty, and America about fifteen, nominal species. Several South American species are intimately related to the two North American ones, and may prove to be only climatic races of the same species; thus, _erythrocnemis_, Gray (Hand List, p. 32, No. 305) may be the intertropical form of _fuscus_, and _chilensis_, Ph. and Landb. (Hand List, No. 314), that of _cooperi_. But the material at my command is too meagre to decide this. [Illustration: =26588=, ♀. ¼ _Nisus cooperi._] [Illustration: =26588=, ♀. ¼ _Nisus cooperi._] [Illustration: =10759=, ♂. ½ _Nisus fuscus._] In consequence of the insufficient material for working up the South American species, I shall omit them all from the following synopsis of the North American species and races.[82] Species and Races. COMMON CHARACTERS. _Adult._ Above bluish slate-color; the tail with obscure bands of darker, and narrowly tipped with white. Beneath transversely barred with white and pinkish-rufous; the anal region and crissum immaculate white. _Young._ Above grayish umber-brown, the feathers bordered more or less distinctly with rusty; scapulars with large white spots, mostly concealed; tail-bands more distinct than in the adult. Beneath white, longitudinally striped with dusky-brown. 1. =N. fuscus.= Middle toe shorter than the bare portion of the tarsus, in front; tarsal scutellæ fused into a continuous plate in the adult male. Tail nearly even. Top of head concolor with the back; tail merely fading into whitish at the tip. Concealed white spots of the scapulars very large and conspicuous. Wing, 6.45–8.80; tail, 5.70–8.20; culmen, .40–.60; tarsus, 1.85–2.25; middle toe, 1.10–1.55. _Hab._ Whole of North America and Mexico. 2. =N. cooperi.= Middle toe longer than the bare portion of the tarsus, in front; tarsal scutellæ never fused. Tail much rounded. Top of the head much darker than the back; tail distinctly tipped with white; concealed white spots of the scapulars very small, or obsolete. Wing, 8.50–11.00; tail, 7.50–10.50; culmen, .60–.80; tarsus, 2.10–2.75; middle toe, 1.30–1.85. _Hab._ Whole of North America and Mexico.