The diagram on the opposite page represents the geological periods in their regular succession, and the approximate time at which all the types and all the classes of the Animal Kingdom were introduced; for there is still some doubt as to the exact period of the introduction of several of the classes, though all geologists are agreed respecting them, within certain limits, not very remote from each other, according to geological estimates of time. RADIATES. Polyps. Acalephs. Echinoderms. T | Present,.........|.........|...........|...... E | | | | R | Pliocene,........|.........|...........|...... T | | | | I | Miocene,.........|.........|...........|...... A | | | | R | Eocene,..........|.........|...........|...... Y | | | | | | | S | Cretaceous,......|.........|...........|...... E | | | | C | Jurassic,........|.........|...........|...... O | | | | N | Triassic,........|.........|...........|...... D | | | | A | Permian,.........|.........|...........|...... R | | | | Y | Carboniferous,...|.........|...........|...... | | | P | | | | R | Devonian,........|.........|...........|...... I | | | | M | Silurian,.....Polyps. Acalephs. Echinoderms... A | R | Azoic. Y |