The relationships of this well-marked genus are, to _Accipiter_ on the one hand, and _Elanus_ on the other; nearest the former, though it is not very intimately allied to either. I cannot admit the subgenera proposed by various authors (see synonomy above), as I consider the characters upon which they are based to be merely of specific importance, scarcely two species being exactly alike in the minute details of their form. The species are quite numerous, numbering about twenty, of which only about four (including the climatic sub-species, or geographical races) are American. North America possesses but one (_C. hudsonius_, Linn.), and this, with the _C. cinereus_, Vieill., of South America, I consider to be a geographical race of _C. cyaneus_ of Europe. The birds of this genus frequent open, generally marshy, localities, where they course over the meadows, moors, or marshes, with a steady, gliding flight, seldom flapping, in pursuit of their food, which consists mainly of mice, small birds, and reptiles. Their assault upon the latter is sudden and determined, like the “Swift Hawks,” or the species of _Accipiter_. In the following synopsis, I include only the three forms of _C. cyaneus_, giving the characters of the European race along with those of the two American ones. Species and Races. =C. cyaneus.= Wing, 12.50–16.00; tail, 9.00–10.70; culmen, .60–.80; tarsus, 2.42–3.25; middle toe, 1.10–1.55. Third to fourth quills longest; first shorter than sixth or seventh; outer four with inner webs sinuated. _Adult male._[78] Above pearly-ash, with a bluish cast in some parts; breast similar; beneath white, with or without rufous markings. _Adult female._ Above brown, variegated with ochraceous on the scapulars and wing-coverts; beneath yellowish-white or pale ochraceous, with a few longitudinal stripes of brown. _Young_ (of both sexes). Like the adult female, but darker brown above, the spotting deeper ochraceous, or rufous; beneath pale rufous, the stripes less distinct. Tail and secondaries without a subterminal band of dusky; lower parts without any markings. Wing, 12.50–15.00; tail, 9.00–10.70; culmen, .60–.75; tarsus, 2.70–2.85; middle toe, 1.10–1.35. _Hab._ Europe … var. _cyaneus_.[79] Tail and secondaries with a subterminal band of dusky; lower parts with rufous markings. Wing, 12.90–16.00; tail, 9.00–10.50; culmen, .65–.75; tarsus, 2.90–3.25; middle toe, 1.20–1.55. Lower parts with scattered irregular specks, or small cordate spots, of reddish-rufous. _Hab._ North and Middle America … var. _hudsonius_. Wing, 12.40–14.50; tail, 8.50–10.50; culmen, .62–.81; tarsus, 2.42–3.00; middle toe, 1.20–1.50. Lower parts with numerous regular transverse bars of reddish-rufous _Hab._ South America …