986 The Perissodactyla. | [Nov, 
No undoubted connecting forms between these sub-orders 
have been discovered, although they approximate at various 
length. In the same genus the distal extremity of the asta 
galus is somewhat convex, and the facet for the cuboid bone is — 
large, somewhat as in the hippopotamus; but the angle sep 
rating the two facets is diagonal, and not transverse; so th 
form a ginglymus, as it does in the Artiodactyla. Ina few ie 
stances some Artiodactyla have teeth which resemble those of 
the Perissodactyla; for instance, the genus Listriodon. Bott 
sub-orders probably arose from an undiscovered common at 
cestor, which was a member of the order Amblypoda. It was 
probably a type with tubercular molars, and belonged to 
_ Puerco epoch. An approach to this theoretical type is 1 
by the Pantolestidz, whose molars are bunodont, the superior 
molars being tritubercular (with two intermediates); but 
form of the extremities (the posterior only is known) is that 
the Diplarthra. The hypothetical Amblypoda with bunodott 
molars I have regarded as a sub-order, and have names 
Hyodonta.* ; 
The opinion has been expressed by Schlosser that the 
tion of the Diplarthra, or alternate-wrist-and-ankle-jointed 
lates, has been directly from the Taxeopoda, or straight 
wrist-and-ankle-jointed Ungulata, without intervention 
Amblypoda. The Periptychidz have been cited as the 
ancestors of the Artiodactyla, and the Phenacodo 
cestors of the Perissodactyla. I do not agree with 
i 
in both the fore and hind feet. This rotation has resu 
or later in the loss of the internal digit (thumb and | a 
from both extremities. In the history of this sliding 
of the first row, the outside element of the row has oe 
ceded in time the inside element. The Amblypo™ ' 
* Proceeds, Amer. Philosoph. Society, 1882, 446 : 
