We shall now turn our attention to next to the highest class of Vertebrate animals--the birds. They and the mammals are the only warm-blooded animals. What is their ancestry? From what original stock did they branch off? The oldest-known bird lived during the Jurassic period, and it was so decidedly reptilian in character as to render it practically certain that birds are specialized descendants of certain Mesozoic reptiles, though not, as might be supposed, of the flying reptiles. The few known specimens of the Jurassic birds were found in the famous lithographic limestone quarries of Bavaria. At least two of the specimens are in a marvelous state of preservation, with practically the whole skeleton intact and almost perfect impressions of the feathers on the rock. That the creature was really a bird is proved not only by its feathers, but also its beak, brain, limb bones, and feet. Among the reptilian characters are its long, vertebrated tail, teeth set in sockets, and long claws on the wings. This reptilian bird was about the size of a small crow. By late Cretaceous time the birds made notable evolutionary progress and they became diversified, more than thirty species being known from Cretaceous rocks. These were distinctly more modern in structure and appearance than the Jurassic bird. The only important reptilian characteristic still retained was the possession of teeth. The tail had become much shortened and the brain was still relatively smaller than in modern birds. One type, about nine inches high, was a powerful flier, as shown by the strong keel and wing bones. Another important Cretaceous type was almost wholly a water dweller, with powerfully developed legs used in swimming. Its teeth were set in grooves instead of in sockets, thus indicating degeneration of tooth structure. This type was notable for its size--five to six feet in length. [Illustration: Fig. 65.--An early type of bird with teeth. This bird grew to a height of about nine inches in Cretaceous time, millions of years ago. (Restored by Marsh.)] During the early part of the Cenozoic era birds became still more advanced and numerous, with many modern groups represented. Some of the more primitive types were, however, still left over during the Tertiary, as, for example, a toothed bird, in which the teeth were merely dentations of the bill, thus being the most degenerate of all types of tooth structure.