1881.] 377 icone, 
The Systematic Arrangement of the Order Perissodactyla. By H. D. Cope. 
(Read before the American Philosophical Society, Aprit 15, 1881.) 
PERISSODACTYLA. 
This, the second great order of the ungulate Mammalia, naturally occu- 
pies a position between the Ambdblypoda and the Artiodactyla. Its lower 
forms are more specialized in the structure of the feet than the Amblypoda, 
while its highest types do not reach the perfection of structure seen in the 
Artiodactyla. This is particularly indicated by the form of the astragalus, 
which has but one, the tibial trochlea, and never displays the distal one 
characteristic of the cloven-footed families. The Perissodactyla occupy, 
as regards their dentition, a position parallel with the Artiodactyla. They 
are always superior in dental complication to the Proboscidia and the suil- 
line Artiodactyla, but only one series, that of the horses, reaches the com- 
