This lower lake was of later (or upper) Eocene age, and the extinct animals preserved in its ancient bed appear to resemble more nearly those of the famous Paris basin, referred to in the beginning of this chapter, than any yet discovered in America. But the basin north of the Uinta Mountains, where alone the Dinocerata had been found, offered so inviting a field that, in the spring of 1871, Professor Marsh began to explore it systematically. He organised an expedition, with an escort of U.S. soldiers, and the work continued during the whole season. In this way a large collection was secured. Explorations were continued in the spring of the following year, which resulted in the discovery of the type specimen of the Dinoceras mirabile. Another expedition was organised in 1873, also with an escort of soldiers, and a great many specimens were collected. These researches were continued during 1874, and again in 1875, with good results. Since then various small parties have been equipped and sent out by Professor Marsh to collect in the same region of the "Bad Lands;" and, finally, during the entire season of 1882, the work was vigorously prosecuted under his direction, and afterwards under the auspices of the United States Geological Survey. This brief account of the difficulties and hardships encountered by Professor Marsh and his companions, for which we are indebted to his exhaustive monograph, will serve to give some idea of the nature of those labours, undertaken in the cause of Science, which he has brought to so successful an issue. * * * * * In the country east of the Rocky Mountains, including the states of Dakota, Nebraska, Wyoming, and part of Colorado, Professor Marsh has discovered the remains of yet another strange group of large quadrupeds. The best known of these is Brontops, of which the skeleton is seen in Fig. 45. These animals lived after the Dinocerata, namely, in the Miocene period, and were the largest American mammals of that period. They constitute a distinct family more nearly allied to the rhinoceros than to any other living form. The skeleton on which Fig. 45 is founded was the most complete of any yet discovered by Professor Marsh. Portions of it were exhumed at different times, but it was first found in 1874. Our artist has made the restoration seen in Plate XV. from this skeleton, as figured by Professor Marsh. [Illustration: Plate XV. A HUGE EXTINCT MAMMAL FROM NORTH AMERICA. BRONTOPS ROBUSTUS.]