The class Aves is most conveniently divided into two subclasses: 1. Archaeornithes. 2. Neornithes.
Subclass I. Archaeornithes.
The only form referred to this subclass of extinct birds is Archaeopteryx[96], the earliest known bird. In this animal the skeleton does not seem to be pneumatic. The cervical and trunk vertebrae are generally thought to be flat, certainly their articulating surfaces are not saddle-shaped. There is no long compound sacrum as in modern birds. The tail is longer than the whole body, the caudal vertebrae are twenty in number, they gradually taper as traced away from the trunk, and each bears a pair of feathers. The posterior caudal vertebrae are not united together to form a pygostyle. The upper jaw bears thirteen pairs of conical teeth, planted in distinct sockets in the maxillae and premaxillae, but the mandible has only three pairs. The presence of these teeth forms the most essential difference between the skull of Archaeopteryx and that of modern birds, and the fact that they occur on the premaxillae renders it improbable that a horny beak was present. There is a ring of ossifications in the sclerotic. The ribs do not show uncinate processes, and articulate with the vertebrae by single heads not divided into capitula and tubercula. Abdominal ribs appear to have been present. The furcula is large, and the scapula has a well developed acromion. The sternum is unknown. The radius and ulna are approximately equal in size. In the manus the first, second and third digits[97] are present, each terminated by a claw. The second digit is considerably the longest, while the third includes four phalanges. The three bones of the pelvis probably remained distinct throughout life. The tarsals are ankylosed respectively to the tibia and metatarsals as in other birds. The metatarsals are ankylosed together, and the pes has four digits.
Subclass II. Neornithes.
To this subclass may be referred all known birds except Archaeopteryx. They all agree in having a short tail whose component vertebrae are commonly ankylosed together forming a pygostyle. The three metacarpals do not all remain distinct. The bones of the pelvis are ankylosed together, and to a large though variable number of vertebrae. There are three orders, the Ratitae, Odontolcae, and Carinatae.
Order 1. Ratitae.
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:—