1881.] . 399 
[Cope. 
The characters of the genera are as follows : 
I. Paleotheriine. Bicipital groove of humerus simple; teeth without 
cementum. 
a, One or more internal tubercles of superior molars distinct. 
External Vs of superior molars not well distinguished externally......... 
Anchilophus. 
External Vs separated by a vertical rib; intermediate tubercles not con- 
necting fore and aft... 0.0... ...cc ccc e cece eee eee Paloplotherium. 
External Vs separated ; intermediate tubercles extended fore and aft...... 
Anchippus. 
aa, Internal tubercles of superior true molars continuous with the 
transverse ridges, 
Inferior molars with two Vs only; lateral toes large.........Paleotherium. 
Inferior molars with distinct internal tubercles ; lateral toes small; a short 
fifth metacarpal...... Shais hdd eeienieslocka-wenhweseeee Mesohippus. 
Inferior molars with cusps at the inner extremities of the Vs; lateral toes 
small; no fifth metacarpal................ teseeveeee Anchitherium, 
Il. Aippotheriine. Bicipital grooove of humerus double; molars with 
cement in the valleys. (Intermediate tubercles connected fore and 
aft. ) 
a. One or more internal tubercles of superior molars distinct. 
Inner lobes of inferior molars enlarged...........eeeeee005 Hippotherium. 
aa. Internal tubercles of molars not distinct, 
Inner lobes of inferior molars enlarged.......... MRO eee eas Protohinpus. 
The genera of this family are generally of less antiquity than those of 
the Chalicothertide, and they range from the Middle Eocene to the Plio- 
cene. Paloplotherium is found in the Middle Eocene, and is, as might have 
been anticipated, more nearly allied to the Chalicothertid@ than any other 
genus of this family. Propaleotherium is not far removed from it. An- 
chilophus is upper Eocene, and is allied to the genus just named, and also 
to Puchynolophus among the Lophiodontide. These early genera consti- 
tute by their similarity, the bond of connection between the three families 
which in their later and specialized forms are very different from each 
other. Paleotherium is chiefly found in the Upper Eocene, and Mesohip- 
pus is only known from the White river or Oligocene, an age between 
Eocene and Miocene. <Anchitherium commences in the Middle Miocene 
and has Anchippus for a cotemporary. Hippotherium existed only in the 
latter part of the Miocene Epoch, consistently with the greatly specialized 
structure of its limbs and teeth, and the nearly allied Protohippus lived 
with it; while in Europe a species with the same type of molar teeth is 
found in the Pliocene epoch (Forsyth-Major).. These forms were cotem- 
porary with the Hquide, which outlived them. They have many points of 
resemblance to that family, but nevertheless remain at a considerable inter- 
val from them in the structure of the feet. 
