ARTIODACTYLA a7 
<3 
of the astragalus divided into two nearly equal facets, one for the 
navicular and the other for the cuboid bone. The caleaneum with 
an articular facet for the lower end of the fibula. Stomach almost 
always more or less complex. Colon convoluted. Ccum small. 
Placenta diffused or cotyledonary. Mamme few and inguinal, or 
numerous and abdominal. 
In treating of many sections of mammals, it is only from the 
existing species that our characters and classification can be derived, 
and to these chiefly our observations upon the group must be 
directed, many of the extinct forms being so little known that they 
can only be referred to incidentally. With the Ungulata, however, 
it is quite otherwise. The history of the Artiodactyla throughout 
the Tertiary period is now well known, and throws great light upon 
the position and relations of the existing groups. 
The principal modifications which have taken place in the type 
from its earliest known and most generalised manifestation have 
been the following :— 
1. As regards the teeth. Assumption by the grinding surfaces 
of the molar teeth either of a bunodont or of a selenodont form. 
Moditieation of the latter from a brachydont to a hypsodont type. 
Loss of upper incisors. Development of canines into projecting 
tusks. Loss of anterior premolars. 
2. As regards the limbs. Reduction of the ulna from a complete 
and distinet bone to a comparatively rudimentary state, in which it 
coalesces more or less firmly with the radius. Reduction of the 
fibula till nothing but its lower extremity remains. Reduction 
and final loss of external pair of digits (second and fifth), with coal- 
escence of the metapodial bones of the two middle digits. Union 
of the naviewlar and cuboid, and sometimes the ectocuneiform, 
bones of the tarsus. 
3. Change of form of the odontoid process of the axis vertebra 
from a cone to a hollow half-eylinder. 
4. Development of horns or antlers on the frontal bones, and 
gradual complication of form of antlers. 
3. Ry inference only, increasing complication of stomach with 
ruminating funetion superadded. Modification of placenta from 
simple diffused to cotyledonary form. 
The primitive Artiodactyles, with the typical number (44) of 
incisor, canine, and molar teeth, brachydont molars, conical odon- 
toid process, four distinct toes on each foot, with metapodium and 
all carpal bones distinct, no frontal appendages, and (in all proha- 
bility) simple stomach and diffused placenta, were separated at a 
very early period into Bunodonts and Selenodonts, although there 
is evidence of intermediate forms showing a complete transition 
from the one modification to the other. These and other fossil 
forms so completely connect the four groups—Suina, Tylopoda, 
