832 SLMGIDILTS, ROU, SI QIQIGIN A 
orbits that are open behind; the multitubercular teeth indicate 
that the animal was omnivorous in its diet; the brain was small 
and smooth, devoid of deep convolutions such as exist in most 
of the mammalian brains. 
Periptychus was a very similar form from the same horizon as 
Phenacodus. 
Pleuraspidotherium and Orthaspidotherium are forms from the 
lowest Eocene, Cernays, of France; they are similar in the 
essential features to the American Condylarthra, and show that 
the group was widespread in its geographical range, as might be 
expected from its generalized characters. 
Starting from the Condylarthra, with its generalized dentition 
and five-toed feet, there were developed two lines of the Ungu- 
lata, which include all of the living and extinct forms. In one 
line the weight of the body is borne on the three middle digits 
of the feet, or, in the more advanced forms, on the middle one 
of all the digits. These forms were called by Owen, in 1849, 
the Perissodactyla. hey are generally referred to as the ‘‘ odd- 
toed”’ animals, with the idea that there is always an uneven 
number of toes on the feet, but this is erroneous, as some mem- 
bers of the group have four toes on the feet; the essential thing 
is that only three of them take any part in supporting the 
body, the other being a rudiment from the original pentadactyl 
arrangement. The next group of the Ungulata is the Arto- 
dactyla, the forms with an even number of toes supporting the 
body. There are, of course, many points in the structure of 
the two groups that are correlated with the development of the 
toes; thus, in the first group there are never any horns devel- 
oped on the parietal bones, and the horns are never paired, but 
there are horns developed on the median line on the nasal bones, 
as in the rhinoceros. There is no living form of the Perissodac- 
tyla that ruminates ; there is a characteristic number of dorsal 
vertebre for each group; the astragalus of the two groups has 
a very different form, and there is always a third trochanter on 
the femur of the odd-toed forms. 
Osborne, in Part I of his memoir on the Extinct Rhinoce- 
