Instead of recognizing, as before, a subclass in the Ratitae of Merrem, Nitzsch now reduced them to the rank of an order under the name " Platysternae," placing them between the " Gallinaceae " and " Grallae," though admitting that in their pterylosis they differ from all other birds, in ways that he is at great pains to describe, in each of the four genera examined by him - Struthio, Rhea, Dromaeus and Casuarius.
A diagnosis covering all the Ratitae (struthio, rhea, casuarius, dromaeus, apteryx and the allied fossils dinornis and aepyornis) would be as follows - (i) terrestrial birds without keel to the sternum, absolutely flightless; (ii) quadrate bone with a single proximal articulating knob; (iii) coracoid and scapula fused together and forming an open angle; (iv) normally without a pygostyle; (v) with an incisura ischiadica; (vi) rhamphotheca compound; (vii) without apteria or bare spaces in the plumage; (viii) with a complete copulatory organ, moved by skeletal muscles.
The common cassowary (Casuarius galeatus) stands 5 ft.
The mooruk, or Bennett's cassowary (Casuarius Bennettii), is a shorter and more robust bird, approaching in the thickness of its legs to the moas.
In the north it is influenced, of course, by its proximity to Papuasia, whence there is a considerable admixture of genera which do not proceed beyond the tropics, and of these Casuarius is a striking example.