MAMMALIA. 519 
second and fourth digits also go, though their metacarpals 
remain as the two splint-bones down the hinder borders of 
the large and elongated third metacarpal or “ cannon-bone,” 
which bears the third digit and the single hoof. Just as in 
origin the cannon-bone of the ox is formed of two meta- 
carpals and that of the horse is one, so they can be imme- 
diately distinguished by the double hinge-joint at the distal 
extremity of the former and the single hinge-joint on that 
of the latter. 
In the hind-limb the femur is recognised by the presence 
in the horse of a ¢hird trochanter on its outer border, and 
the tibiofibula or tibia, carrying the fused remnant of the 
fibula, will be seen in the ox to have three articular facets 
at its distal extremity. The two larger articulate with the 
astragalus, as in the horse, but the small outer one articu- 
lates with a small condyle on the calcaneum. The astragalus 
in the horse has a flat facet for the navicular below it, but 
that of the ox has a hinge-joint with the naviculo-cuboid 
bone below it, which gives ita double appearance, a hinge- 
condyle at each end. In other words, the horse has only 
a crurotarsal joint, as in most mammals, but the ox has a 
certain amount of intertarsal movement as well as the 
crurotarsal. Of the distal tarsals the navicular and cuboid 
fuse across in the ox to form a naviculo-cuboid, whereas in 
the horse the navicular commonly fuses with the ecto- 
cuneiform below it, or remains distinct, but never fuses with 
the cuboid. There is usually a small middle cuneiform in 
the horse over the inner splint bone (digit two). The 
digits of the hind-foot are modified in a closely similar way 
to those of the fore-foot. : 
The metacarpal ‘‘cannon-bone” of the ox is distinguished from 
the metatarsal by the much shallower median groove in the former, 
and the metacarpal ‘‘cannon-bone” of the horse is flattened from the 
front behind, whereas the metatarsal is round in cross section. 
If it be remembered that the horse’s limb is formed from 
hypertrophy of one digit and the bones in the main axis 
above it, whereas that of the ox is really bilateral or formed 
from two digits and the bones above them, which are only in 
later geological times fusing together to form one, it is easy 
to account satisfactorily for the persistent calcaneo-fibular 
joint, for the fusion across the middle line of navicular and 
