climbing (woodpecker), birds of prey (Fig. 77) (eagle), and running birds (Fig. 73) (ostrich). This classification is only useful as a means of superficially arranging the eight thousand species of birds known to naturalists. An examination of the skeleton, however, offers us only three orders, the first of which, possessing a long bony tail, is represented by a single genus, the extinct Archeopteryx; the second, in which the breast-bone is furnished with a ridge or keel, whence the name Carinata, includes ordinary birds; the third order has no keel on its breast-bone, and is represented by the Ostrich, Emeu, etc. Although birds differ considerably from reptiles, yet they agree with one another in very many important characters, such as peculiarities in the structure and articulation of the skull and backbone, and in the structure and articulation of the lower jaw with the skull. The brain, heart, and great cavities of the body are alike in these orders. These common characters, as well as others, seem to warrant the union of birds and reptiles in a natural group, of which the birds are the most advanced in specialized structure. This view is confirmed by the embryology and fossil remains of birds and reptiles. The Ostrich family, among existing birds, perhaps approaches nearest the reptiles. Nevertheless, the gap was very great between these classes. By the discovery of the existence in past time of the Dinosaurian reptiles and Archeopteryx, this gap has been bridged over. The Archeopteryx offers the only known example of a bird with a bony tail, that is, among adult birds, as birds, when in an embryonic condition, have quite as much of a tail as a turtle. The bones of the hand (metacarpal), in the Archeopteryx, are not soldered together (anchylosed) as in birds, but remain distinct. In these marked and important peculiarities the Archeopteryx agrees with reptiles and differs from birds. The Archeopteryx, in fact, is a reptile-like bird. In the Compsognathus, a fossil reptile,
