A much smaller flightless bird, the dodo, became extinct in modern time. This former inhabitant of Mauritius and other islands of the Indian Ocean was related to the doves and pigeons, and had lost its power of flight through disuse of the wings. It was a clumsy, defenseless bird weighing possibly as much as fifty pounds. Actual remains are few and incomplete, and descriptions published by the explorers who knew the bird two centuries ago are not entirely trustworthy. In the Pleistocene Rancho la Brea beds of California the largest of all prehistoric flying birds has been found, a vulture bearing the name of _Teratornis_. Re-assembled skeletons show them to be slightly bigger than existing condors. ANCESTORS OF THE MAMMALS The monotremes or egg-laying types of mammals such as the duck-bill and spiny anteaters which now inhabit Australia are almost unknown as fossils. Marsupials, the next higher living group, which includes the opossum and kangaroo, appeared at the end of Cretaceous time along with the placentals or higher mammals which dominate the history of the Cenozoic era. Nevertheless, there are a few teeth and jaws from rocks of Triassic and Jurassic age to indicate that small mammals, from the size of mice to slightly larger than rats, existed throughout most of the Age of Reptiles. There is no complete skeleton of any of the earlier forms, and little is known of their relationships either with living orders of mammals or with probable ancestors among the reptiles. The record becomes somewhat clearer toward the end of the era but it is obscured again by the great disturbances which followed. Looking back among earlier land animals for the origin of the first mammalian stock it is necessary to go as far as Permian or even Carboniferous time. Reptiles then living had many structural features in common with mammals, and mammal-like forms continued to flourish until late in the Triassic. An interesting group of such animals, named therapsids, was one of the earliest reptilian stocks to appear, and is well known from fossils found in the Red Beds of Texas and New Mexico, in Europe, South Africa, and Asia. Quite a variety of types is included in this group, with many advances in dentition, and modifications of the skull, limbs and pelvic construction which strongly suggests a relationship to the mammals. [Illustration: Murals Over Fossil Exhibits, Hall of Mammals Top: Eocene; Protylopus, Tanyorhinus, Patriofelis, Uintatherium, Turtle, Crocodile, Eohippus. Middle: Upper Oligocene; Mesohippus, Merycoidodon, Hoplophoneus, Metamynodon, Poebrotherium, Trigonias. Bottom: Pliocene; Teleoceras, Turtle, Synthetoceras, Amebelodon, Teleoceras.] THE AGE OF MAMMALS