Among the exhibits of the Denver Museum of Natural History may be seen complete skeletons of extinct horses, rhinoceroses, titanotheres, and chalicotheres representing the perissodactyls. _Moropus_ was one of the chalicotheres, an exceptional family which never became very prominent although it had a prolonged history and persisted in Europe and Asia after its extinction in North America. The family is grouped with the ungulates because of many similarities found in the molar teeth, skulls, and other parts of the skeletons, but the toes were provided with claws instead of hoofs. The use of these claws is somewhat of a puzzle: possibly for defense against carnivorous enemies, for dragging down branches in order to obtain food, or for digging roots which may have been an important part of the diet. Titanotheres are represented in our collection by the skeletons of the large, horned type which was the last of the race and destined to extinction by the middle of the Oligocene period. Smaller hornless varieties of Eocene time are illustrated by skulls. This family of ungulates had an unprogressive dental equipment, and a small brain in a flattened skull. The molar teeth readily distinguish the group from other ungulates and enable us to trace the relationship between earlier and later varieties. These teeth were of a type which is soon destroyed by wear, and it is evident that the animals survived only so long as their environment provided them with an abundance of soft vegetation. [Illustration: Titanotheres of Oligocene Time The name of these animals refers to the large size though they were greatly exceeded in bulk by the mastodons and mammoths of later periods. Ancestral titanotheres, dating back to the Eocene, were hornless animals of much smaller size. These splendid specimens were obtained in Weld County, Colorado.] The large assortment of rhinoceros material provides an idea of the great abundance and variety of forms in this family which was once prominent in North America but no longer among the inhabitants of that continent. Some of the mounted skeletons have been restored on one side to show how these animals appeared in the flesh. Of the even-toed ungulates there are also several types illustrated by complete skeletons. _Merycochoerus_, the subject of one of our mounted groups, represents the oreodonts, a large family of mammals whose history begins with the Upper Eocene and ends in the Lower Pliocene. The oreodonts were small animals, rather pig-like in form and quite common in the western plains region shortly after the time of the titanotheres. Ancient swine are represented in our exhibits by two mounted skeletons which were obtained from northeastern Colorado, where the bones were found associated with rhinoceros and titanothere remains. Some of these animals were of very large proportions, and the entire family is commonly known as the “giant pigs.”