Other skeletons include the saber-tooth tiger, characterized by the long curved upper canine teeth which undoubtedly were used for stabbing and slashing, and the dire wolf, the smaller of the two which are facing the sloth. The artist’s reconstruction of this scene also shows the great vulture, Teratornis, which is the largest known bird of flight.] Over-specialization in the production of tusks appears to have been the principal factor in the downfall of the mammoths. The large size of the animals and the difficulties of finding sufficient food to sustain life must have been a serious handicap at times, but their ability and inclination to travel over long distances enabled some of them to find tolerable living conditions until the end of the Glacial Period. They are now extinct and the nearest living relatives are the elephants, somewhat reduced in size of tusks and body but otherwise very similar. There are many other tribes of mammals whose ancient history is partially known though broken by periods of time for which there is no fossil evidence. All have undergone changes in which various forms and degrees of specialization are featured; this general process is best revealed by the horses and elephant-like animals which have left a clearer record. For other groups the story would differ but little except as to names and specific details. THE RANCHO LA BREA FOSSIL PITS The La Brea tar pits, as they are often called, provide a remarkable record of Pleistocene life in southwestern North America. Scattered over an area of about thirty acres just off Wilshire Boulevard in Los Angeles, these bone deposits were known, as far back as 1875, to contain the remains of prehistoric animals. It was not until 1905, however, that their value was recognized by paleontologists. In that year the University of California began an investigation, and excavations were carried on at intervals by various institutions during the next ten years. A great deal of material was acquired by the Los Angeles Museum of History, Science, and Art, where many skeletons, skulls, and other interesting specimens have been placed on exhibition. The pits have the form of small craters formed by the seeping of oil from the underlying rocks. The seeps appear to have been active during part of the Pleistocene period but apparently not at the beginning. The oil is rich in asphalt which has served as a preservative for the bones, and owing to its sticky properties has been an effective animal trap for thousands of years. The fossil beds at present are of oil-soaked earth and sand. In past times there must have been a greater percentage of oil, often concealed by a layer of dust or pools of water. The large number of carnivorous animals found in the deposits suggests that they were attracted by the cries and struggles of creatures wandering carelessly into the asphalt and serving as live bait to keep the traps in continuous operation.