Paleontology deals with the History of Life. Its time is measured in geologic epochs and periods, in millions of years instead of centuries. Man, by this measure, is but a creature of yesterday--his "forty centuries of civilization" 1] but a passing episode. It is by no means easy for us to adjust our perspective to the immensely long spaces of time involved in geological evolution. We are apt to think of all these extinct animals merely as prehistoric--to imagine them all living at the same time and contending with our cave-dwelling ancestors for the mastery of the earth.