The Perissodactyla. 1073 
1887] 
i The genera of Equip# are but two in number, and they are 
_ defined as follows : 
Internal lobes of superior molars sub-equal ; ......sesseeseeees Hippidium. 
= Anterior internal lobes af superior molars much larger 
than the posterior; ; guus. 
_ The genus Hippidium is extinct, and its species have been thus 
és far found only in North and South America, in beds of Pliocene 
and upper Miocene age. Equus made its appearance during the 
l former period, and is represented by several existing species. 
The Equidze adds another evidence of greater specialization 
than the Palzotheriide in the structure of its feet—7e., the 
Until recently, a puzzle to me. I have endeavored to show that 
€ development of the tongue at the extremity of the meta- 
 Podials of the Diplarthra was due to the impacts of the terminal 
phalanges often repeated, on hard ground, together with the 
Compression of the surface on each side the keel by the flexor 
ttndons with their sesamoid bones. But this did not account 
Diplarthrous ungulates the phalanges are flexed at right — 
Mer; n the 
_ ver. This fact accounts for the late appearance 
_ lime of its effect on the end of the metapodial bone. 
‘dd here that the acuteness and narrowness of the keel is partly 
~ to the movement of torsion conveyed throughout all the 
x ones of the feet at the moment of arrest by the ground, as 
"Ened to in the opening pages of this paper. 
T must here describe another effect of torsion © 
“the 
oe 
I must 
Re 
mals extended inwards from the inner towards 
the foot, so as to abut on the carpal or tarsal 
thedigit next external to them, This is due to pressure through 
