1 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 
Horses they are abundantly coated with cement. The shaft of the 
ulna is reduced to a mere style anchylosed throughout nearly its 
whole length with the radius, and appearing to be little more than a 
ridge on the surface of the latter bone until it is carefully examined. 
The front toes are still three, but the outer ones are more slender 
than in Anchitherium, and their hoofs smaller in proportion to 
that of the middle toe; they are, in fact, reduced to mere dew- 
claws, and do not touch the ground. In the leg, the distal end of 
the fibula is so completely united with the tibia that it appears to be 
a mere process of the latter bone, as in the Horses. 
In Lquus, finally, the crowns of the grinding-teeth become longer, 
and their patterns are slightly modified; the middle of the shaft 
of the ulna usually vanishes, and its proximal and distal ends anchy- 
lose with the radius. The phalanges of the two outer toes in each 
foot disappear, their metacarpal and metatarsal bones being left as 
the “splints.” 
The Hipparion has large depressions on the face in front of the 
orbits, like those for the “ larmiers ” of many ruminants ; but traces 
of these are to be seen in some of the fossil horses from the Sewalik 
Hills; and, as Leidy’s recent researches show, they are preserved 
in Anchitherium. 
When we consider these facts, and the further circumstance that 
the Hipparions, the remains of which have been collected in immense 
numbers, were subject, as M. Gaudry and others have pointed out, 
to a great range of variation, it appears to me impossible to resist 
the conclusion that the types of the Anchitherium, of the Hippa- 
rion, and of the ancient Horses constitute the lineage of the modern 
Horses, the Hipparion being the intermediate stage between the 
other two, and answering to B in my former illustration. 
The process by which the Anchitherium has been converted into 
Equus is one of specialization, or of more and more complete deyi- 
ation from what might be called the average form of an ungulate 
mammal. In the Horses, the reduction of some parts of the limbs, 
together with the special modification of those which are left, is 
carried to a greater extent than in any other hoofed mammals. 
The reduction is less and the specialization is less in the Hippa- 
rion, and still less in the Anchitherium; but yet, as compared 
with other mammals, the reduction and specialization of parts in 
the Anchithertum remains great. 
Is it not probable then, that, just as in the Miocene epoch, we 
find an ancestral equine form less modified than Hquus, so, if we 
go back to the HKocene epoch, we shall find some quadruped related 
to the Anchitherium, as Hipparion is related to Equus, and conse- 
quently departing less from the average form ? 
I think that this desideratum is very nearly, if not quite, supplied 
by Plagiolophus, remains of which occur abundantly in some parts 
of the Upper and Middle Eocene formations. The patterns of the 
grinding-teeth of Plagiolophus are similar to those of Anchithe- 
vium, and their crowns are as thinly covered with cement ; but the 
grinders diminish in size forwards, and the last lower molar has a 
