Geology tells of a great gap between the highest rocks of the Cretaceous period and the lowest group of the succeeding Eocene period (see Table of Strata, Appendix I.). This gap, or break, testifies to a very long interval of time during which important geographical and other changes took place; and consequently we find in Eocene rocks (at the base of the Cainozoic series) a very different fauna and flora to that which is preserved in the Chalk formation. The researches of Cuvier among the fossils collected from Eocene rocks in the neighbourhood of Paris, especially the Gypseous series of Montmartre, revealed the existence of a very extensive fauna, especially of new types of mammals; and his restoration of the Palæotherium, a tapir-like animal, and other forms, created a vast amount of interest, and greatly stimulated the study of extinct animals. As we have already remarked, the science of palæontology may be said to have been founded by Cuvier (see Introduction, p. 5). But now the scene shifts once more from Europe to the wilds of the Far West. American geologists tell us that a long time ago (during the Eocene period) there was a great tropical lake in the Wyoming territory, on the borders of which roamed, amidst luxuriant vegetation, a large number of strange and primitive quadrupeds, together with many other forms of life. The most wonderful group of animals that haunted the shores of this lake, or series of lakes, was the Dinocerata so fully described by Professor Marsh, in his exhaustive monograph.[40] The name implies that they were terrible horned monsters, but whether Nature provided them with true horns, like those of horned cattle to-day, is at least open to doubt. [40] _The Dinocerata_, a monograph by O. C. Marsh, _United States Geological Survey_, vol. x. Fig. 42 shows the skeleton of one of these, namely, Tinoceras ingens. Its length was about 12 feet without the tail. Its weight, when alive, is calculated to have been six thousand pounds, or about two tons and three quarters. Plate XIV. is a restoration of the Tinoceras, made by our artist, after much consideration and careful study of the valuable cast exhibited in the Natural History Museum at South Kensington, which was generously presented by Professor Marsh (Gallery I. Case MM on plan). In planning this and other restorations, both artist and author have received valuable assistance from Dr. Henry Woodward, F.R.S., Keeper of the Geological Department of the Museum, who is ever ready to help with his great knowledge those who come to consult him.