39 
skull of Zittanothertum bucco, exhibits the horns in their ex- 
treme forms of development, taking up the whole anterior por 
tion of the skull. 
179. THE ANCESTRAL FOUR-TOED HORSE AND THE MODERN 
Horse. Placed side by side are skeletons of the diminutive 
Hyracotherium venticolum, or four-toed horse, 34 hands high, and 
the skeleton of a recent horse, Eguus caballus, 154 hands, be- 
longing to the modern trotting breed. The interval in time 
which separates these types is roughly estimated at two mil- 
lion years. The former skeletonis unique; it comes from near 
the base of the Eocene, and was found by Dr. Wortman, in 
1883. It belongs to the famous Cope Collection which has 
been recently acquired by the American Museum of Natural 
History. This is publicly exhibited for the first time. 
180. EVOLUTION OF THE SKULL OF THE HORSE. This series of 
skulls, probably the most complete which has ever been brought 
together, shows the changes which have transformed the di- 
minutive skull of Hyracothertum venticolum, or four-toed horse, 
into that of the living horse. 
181. EVOLUTION OF THE FOOT OF THE HORSE. This series of feet 
_ is parallel to that of the skulls, and shows the changes through 
which the feet have passed from Hyracotherium venticolum to 
Equus caballus. These changes, which are fully explained 
upon the labels, consist mainly in the enlargement of the 
median toe and the reduction and disappearance of the lateral 
toes. 
182. SKULLS OF AMERICAN FOSSIL RHINOCEROSES. The first 
remains of the Rhinoceroses in America were announced by 
Leidy in 1859. Since then the explorations of Cope have 
brought to light two new forms, and the expeditions of the 
American Museum have procured three new forms, representing 
probably the complete series of Lower Miocene Rhinoceroses. 
They are observed to increase gradually in size, in the compli- 
cation of the grinding teeth, and in the loss of the canine and 
lateral pair of incisors. The last member of the series is an 
ancestor of the two-horned Rhinoceroses which appear in the 
Middle Miocene. a. Aceratherium trigonodum. This is the 
