The Ratitae differ from _Archaeopteryx_ and the great majority of Carinatae in being flightless. The bones are generally not pneumatic, containing marrow instead of air, in the Ostrich however they are very pneumatic. The tail is short and the posterior caudal vertebrae are generally ankylosed together forming a pygostyle. The pectoral girdle has comparatively a much smaller size than in Carinatae, clavicles are small or absent, and the scapula and coracoid lie nearly in the same straight line. The ilium and ischium do not as in Carinatae unite posteriorly, and enclose a foramen except in very old Rheas and Emeus. The quadrate articulates with the cranium by a single head. The vomers unite and form a broad plate, separating the palatines, pterygoids and basisphenoidal rostrum. The anterior limbs are greatly reduced in size or even absent, while the posterior limbs are greatly developed and adapted for running. The tibia and fibula are quite distinct. Many ornithologists agree that the various forms grouped together as Ratitae are not all very closely allied to one another, that they resemble one another mainly in having lost the power of flight, and do not form a natural group. The Ratitae include the following groups:-- _Æpyornithes_[98], huge extinct birds from Madagascar. _Apteryges_, including the Apteryx of New Zealand. _Dinornithes_[99], the Moas, huge extinct birds from New Zealand, and some of the neighbouring islands. _Megistanes_, including the Cassowaries (_Casuarius_) of Australia, New Guinea, and some of the neighbouring islands; and the Emeus (_Dromaeus_) of Australia. _Rheornithes_, including the Rheas of S. America. _Struthiornithes_, including the Ostriches (_Struthio_) now living in Africa, and found fossil in N. India and Samos. _Order_ 2. ODONTOLCAE. This order includes only an extinct N. American bird _Hesperornis_[100]. The jaws are provided with a series of sharp teeth placed in continuous grooves, but the premaxillae are toothless, and were probably sheathed in a horny beak. The rami of the mandible are not ankylosed together in front. The skeleton is not pneumatic. The cervical vertebrae have saddle-shaped articulating surfaces as in ordinary birds, and the thoracic vertebrae are not ankylosed together. The tail is comparatively long, and formed of twelve vertebrae with only slight indications of a pygostyle. The ribs have uncinate processes. The anterior limb is quite vestigial, being reduced to a slender humerus. The posterior limb is very powerful and adapted for swimming. _Order_ 3. CARINATAE.