HYVRACOIDEA. 763 
sometimes a large lower pair; there are a few incisors, but these 
are often small and deciduous. 
There are several entirely extinct families of Perissodactyla, such as— 
Lophiodontidee (Eocene), e. eg. Lophiodon, Hyracotherium, Sys- 
temodon,—a family perhaps ancestral to most of the modern 
Perissodactyla. 
Palotheriidze (Eocene to Miocene), eg. Paleotherium and 
Anchitherium. : 
Other remarkable types—Lamébdotherium, Chalicotherium, Titano- 
therium, of elephantine size, and the specialised Afacrauchenia 
—are referred to distinct families. 
Sub-Order HyracorDEa 
An isolated order of small Rodent-like Ungulates, repre- 
sented by Ayrax (Procavia) and Dendrohyrax, living in 
rocky regions and on trees in Africa and Syria. The 
species (14) are adept climbers. 
The upper incisors have persistent pulps, and are curved 
as in Rodents, but they are sharply pointed, not chisel- 
edged. The outer lower incisors are straight, and have 
trilobed crowns. There are no canines in the second set, 
but the upper milk canine sometimes persists; and there 
is a wide space between incisors and premolars. The back 
teeth are very uniform, and like those of Perissodactyla. 
The milk dentition is 34, the permanent is 5. Ayrax is 
one of the few Mammals in which the first premolar is a 
replacing tooth. The jugal forms part of the glenoid cavity 
(cf. Marsupials). 
In the fore-feet the thumb is rudimentary, the ‘little 
finger is smaller than the median three, which are almost 
equal. In the hind-feet, which are like miniatures of 
those of the rhinoceros, the hallux is absent, ‘and the 
fifth toe is rudimentary. There are no hoofs in the 
strict sense. There are no clavicles. The tail is very 
short. 
The brain is like that of Ungulates. The stomach is 
divided into two parts by a constriction. In addition to 
the short but broad’ cecum, there are two supplemental 
ceca lower down on the. intestine. The testes are 
abdominal. Of the mamme, four are on the groin and 
two are axillary. The placenta is zonary, as in the Pro- 
boscidea and Carnivora. A few extinct forms are known. 
