Cope.] 400 ? [April 15, 
The geographical distribution of these genera, so far as present knowl- 
edge shows, is as follows: 
North America alone—Mesohippus, Anchippus. 
North America and Europe—Anchitherium, Hippothertum, Protohippus. 
Europe only—Anchilophus, Paloplotherium, Paleotherium. 
EQUID&. 
The two genera of this family are distinguished as follows : 
Internal lobes of superior molars subequal..............4.66% Hippidium. 
Anterior internal lobes of superior molars much larger than the pos- 
VOLIOM: os sss di heindaG ios caaung Sec aweees eeu gn ae erauaienies veeee DQuus, 
The genus Hippidium is extinct, and its species have been thus far found 
only in North and South America, in beds of Pliocene age. Hquus made 
its appearance during the same period, and is represented by several exist- 
ing species. 
Besides the reduction in the number of digits, which is carried farther 
here than in any other family of Mammalia, there are several other char- 
acteristics of specialization. Thus in the dentition, the spaces between the 
tubercles are filled with cementum. These valleys are generally deep, owing 
to the prismatic forms of the molars. The cups of the incisors are com- 
pletely developed, and also filled with cementum. There are two bicipital 
grooves of the humerus. The preceding characters are also found in the 
Hippothertine of the Paleotheriide. 
The Equide adds another evidence of greater specialization than the 
latter group in the structure of its feet, 7. ¢., the distal metapodial keels are 
completed forwards, as in most ruminants. 
The similarity of the modifications which have supervened on the Artio- 
dactyle and Perissodactyle lines in attaining their most specialized extreme 
has often been noticed. I repeat them here in tabular form in three 
columns. These show (Table I) the modifications in which the Hguide 
and Bovide are identical or nearly so, which place them at the heads of 
their respective orders; Table II, those in which the Hguid@ are the 
more specialized of the two; and Table III, those in which the Bovide dis- 
plays the highest differentiation. 
