3 1887] _ The Perissodactyla. 993 
8. Toes 3-3; no vertebrarterial canal; M. heniide 
AA. Premolars like molars, with two internal lobes above. 
OLT a ce ea AP) FF 0 000 00s ee condcncerecoesseecueesedeteeseteeuseeséce ‘enodontide . 
10, Toes with digits 3-3; Paleotheriida 
I. Toes with digits 1-1;... Equide. 
The total number of well-determined species of this order is 
_ about two hundred. It was abundantly represented during the 
_ Eocene period, and the recent species are comparatively few. 
_ Itmay be also observed that certain families predominated dur- 
ing certain periods. Thus the prevalent Perissodactyla of the 
Eocene are Lophiodontide and Chalicotheriide; those of the 
Miocene are Rhinocerontide and Paleotheriide. The Ti apiride 
and guide characterize the latest tertiary epochs. A genea- 
logical tree of the order may be constructed as follows: 
Equide. 
Rhinoceridz. Palzotheriide. 
Hyracodontide, Tapiride. Menodontidz. 
Triplopide. Cænopidæ. 
Lophiodontidæ. Chalicotheriidæ. 
_ E modification of known forms (as Lambdotherium in the 
ide, and Hyracotherium in the Lophidontide) would 
~ necessary to obliterate this difference is quite clear. How | 
readily the transverse-crested lower molar can be modified into 
S ing type may be seen by comparing the inferior molars 
Hyracotherium with those of a rhinoceros. 
ao 1. 
ns 2 the species belong to the Eocene period. They range 
moe e Size of a rabbit to that of an ox. They resembled 
ena living animals, the tapirs. 
i. ~~ &enera are characterized as follows: a 
l ‘Exteral lobes of superior molars well separated and little flattened; lobes of 
_ Metior molars scarcely united (Hyracotheriina). 
