The _Circus assimilis_ may be regarded as the commonest of the Harriers inhabiting New South Wales and South Australia; it also occurs, but in smaller numbers, in Van Diemen’s Land. Another Harrier is rather abundantly dispersed over all the localities suitable to its existence in Western Australia, and it is just possible that they may prove to be mere varieties of each other; if such should be the case, the whole of the southern portion of the coast of Australia, from east to west, must be included within the range of its habitat; still, without further evidence in favour of this supposition, I should consider them to be distinct species; and if this opinion is well-founded, the two species will be found to inosculate in the latitude of Spencer’s and St. Vincent’s Gulfs, as, in the collections lately forwarded to me by Mr. Harvey of Port Lincoln, I found two or three individuals precisely identical with those from Swan River. In size the _Circus assimilis_ is but little inferior to the Marsh Harrier (_Circus æruginosus_) of Europe, to which it offers a great resemblance in its habits and economy; being generally seen flying slowly and somewhat heavily near the surface of the ground, evincing a partiality to lagoons and marshy places, situations which offer it a greater variety and abundance of food than any other; the principal part of its food consists of reptiles, small mammalia and birds. I several times observed this species in the lagoons near Clarence Plains in Van Diemen’s Land, as well as in all similar situations in almost every part of New South Wales I visited. I was not so fortunate as to find the nest of this Harrier,—a knowledge of its form and of the colour of its eggs is therefore yet to be ascertained. That it breeds in the localities in which I observed it I have little doubt, from the circumstance of the adults paying regular and hourly visits to the marshes in search of food, which was doubtless borne away to their young. When in a state of quiescence, this species, like the other Harriers, perches on some elevation in the open plain rather than among the trees of the forest; the trunk of a fallen tree, a large stone, or small hillock, being among its favourite resting-places. The sexes offer the usual differences in the larger size of the female; the markings of that sex are also rather less well-defined, and have not so much of the grey colouring as the male.