272° Origin of the Foot Structures of the Ungulates. (April, 
tal! Similar evidence is furnished by the genus Lurytherium of 
the French Eocene. Its posterior foot is modified artiodactyle, 
‘while the anterior is modified perissodactyle. We may assume 
from these facts, that the posterior foot is more subject to the 
influences which tend to produce the bidigital structure than is 
the anterior limb. 
I suspect that the production of a ginglymus in the middle 
of the tarsus, has been due to the use of the posterior limb in 
soft swampy ground. In the absence of this condition, as in a life 
on harder ground than swamp, no ginglymus would be formed. 
The action of an ungulate in walking through deep mud is very 
‘ suggestive. The posterior foot is bent on the leg, 
and the antero-posterior strain of the weight or 
propulsive force, is transverse to its long axis. In 
progression on dry land, the impact is in the 
direction of the length or axis of the foot. The 
obvious effect of a cross strain is to produce 
by degrees greatér and greater mobility of some 
articulation. The one which has yielded is that 
between the two tarsal rows. Another effect of 
walking in swampy ground is to spread the 
digits apart. As the first digit of both feet is 
always of reduced size, there are practically but 
four digits to be considered. The weight falling 
nearly medially on these, would tend to spread 
them equally, two on each side. Thus the same 
cause may have been effective in producing both 
deg ernipssieg the artiodactyle structures. The perissodactyle 
radius of Poésro- structure, so soon as the lateral digits are much 
ro pathy: ot onler reduced, ceases to be favorable for progression 
three-fifths nat size. in soft ground, owing to the liability of the lateral 
ae Bl cx. digits to injury, in following the principal one 
into the asec material, filled with sticks and 
other hard deébris: 
The lowest. existing forms of the Artiodactyla, the Omuivora, 
are universally swamp lovers and livers. So we are told are the 
lower existing Perissodactyla, the tapirs and rhinoceroses. The 
higher types of both orders are dwellers on plains and in forests. 
We do not know the habits of the Eocene Perissodactyla, but I 
doubt their having inhabited muddy ground to the same extent as 
