504 O. C. Marsh—Polydactyle Horses. 
during the whole of Tertiary time, this continent was occupied 
with equine mammals, of many and various forms. Although 
all these became extinct before the discovery of this country, 
their abundant remains mark out the genealogy of the horse in 
an almost unbroken succession of forms. 
we examine the remains of the oldest representatives of 
the horse in this country, we shall find that these animals were 
all polydactyle, and of small size. As the line was continued 
pesos and note especially the changes in the number of digits. 
the group now known is the Hohippus, which had four well 
developed toes and the rudiment of another on each fore foot, 
its appearance. It resembled its predecessor in size, but had 
only four toes in front and three Caeat as shown in the lowest 
series of the diagram. At the top of the Eocene, a third allied 
genus has been found (Hpthippus), which closely resembled 
ohippus in its digits, but differed in its teeth. 
Near the base of the next formation, the Miocene, another 
equine mammal, Mesohippus, occurs. This animal was about 
as large as a sheep, and had three usable toes and the splint of 
another, on each fore foot, with but three toes behind, as shown 
in the diagram. At a somewhat higher horizon, a nearly allied 
genus, Miohippus, has been found, which has the splint bone 
of the outer or fifth digit reduced to a short remnant. In the 
liocene above, a three toed horse (Protohippus) about as large 
as a donkey was abundant, and still higher up a near ally 
of the modern horse, with only a single toe on each foot, 
___ (Pliohippus) makes his appearance. A true Equus, as large as 
__ the existing horse, appears just above this horizon, and the 
___ Beries is complete. | : 
Yale College, New Haven, May 15th, 1879, 
