The ischium and postacetabular ilium originally enclose the ischiadic notch or incisura ischiadica.
The shaft of the pubis runs parallel with that of the ischium, with which it is connected by a short ligamentous or bony bridge; this cuts off from the long incisura pubo-ischiadica a proximal portion, the foramen obturatum, for the passage of the obturator nerve.
They are typically nidifugous; the simple structure of their neossoptiles; quintocubital; compound rhamphotheca; holorhinal nares imperviae; basipterygoid processes; simple articular facet of the quadrate; configuration of the palatal bones, including the large vomer; incisura ischiadica; simple hypotarsus; the thigh muscles; the copulatory organ.
Order Tinamiformes.-Nidifugous, with incisura ischiadica, without pygostyle.
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.