1074 : The Perissodactyla. [Dec 4 
the carpals and tarsals of the second row or the heads of the 
metapodials, which is by the torsion turned from within outwards, 
The pressure thus applied has gradually pressed the heads of the 
metapodials outwards in the manner described. This effect be- 
gan earlier than diplarthrism,’ as it is Seen in the Condylarthra. 
In the preceding discussion of the phylogeny of the Perisso- 
dactyla the descent of genera within families has been described, 
and also the descent of families in their 
entireties has been discussed. The attempt — 
to carry the line of generic succession across 
the boundaries of families has not been gen- 
erally made. The lack of knowledge of such 
intermediate genera is the cause of this 
omission. Were such genera known, the 
definitions of the families would be less _ 
precise than they are. This complete phy- 
logeny has been attempted, however, 1n the 
case of the genus Equus by various authors. 
The first suggestion was made by Cuvier, and 
the first arrangement of genera in the phylo- 
genetic line of the horses was by Kowalevsky. 
His series commenced with Anchitheriu™, 
and had the other members Hipp 
and Equus. To this series Huxley added 
Palzotherium. Later, Marsh added ‘0 
definite terms to the series, Hyco d 
and Hippidium, giving to both, bee 4 
names (Eohippus and Pliohippus), y r - 
posed two other steps (Orohippus an prie ; 
hippus), which were not sufficiently age a 
ized to be since recognizable. The nO: : 
writer determined the identity of paken 
forms, and added the still more primitive genus SY see 
don: He also discovered and defined the Condylarthra, fo 
of which, (Phenacodontid), he announced as the ances a 
all Perissodactyla, horses included? Dr. Wortman 5 > he 
pointing out the double descent of the genus Equus coe 
two lines of Palzotheriide, and indicating the relations of 
* Cuvier and Kowelevsky have shown that in the genera Sus and 
head of the second metapodial is expanded inwards as well as outwards. — 
* Proceeds. Amer. Philosoph. Soc., 1881, p. 178- 
Gaudry, “ Enchaine- 
ments.” 
