520 CHORDATA. 
cuboid, for the “‘ double” astragalus, and for the fused third 
and fourth metapodials. 
The numerous structural resemblances and differences 
in the horse and the ox we may sum up as follows :— 
1. Resemblances of the two types which are due to 
descent from a common mammalian ungulate ancestor. 
These are characters of ordinal rank or the distinctive 
characters of the order Ungulata. The most important 
are the presence of a dentition adapted for a vegetable diet, 
heterodont and diphyodont; the commencing adaptation 
of the limbs for terrestrial locomotion with claws tending to 
assume the condition of hoofs ; little or no clavicle. 
2. Resemblances due to evolution on similar lines since 
the divergence from a common stock. Of these we may 
instance (1) The assumption of a digitigrade locomotion 
and reduction in number of toes. (2) The interlocking of 
carpal and tarsal bones (diplarthrous) connected with the 
increasing size and rapidity of movement on harder ground. 
(3) The expansion of the facial region, correlated with the 
increased size of molar teeth, and the completion of bony 
orbit. (4) The conversion of brachydont teeth into hypso- 
dont, the increased complexity of the enamel ridges and the 
addition of cement. 
3. Differences due to evolution on somewhat distinct 
lines since divergence from the common ancestor. The 
principal of these are (1) The modelling of the limbs in the 
horse, on the one-toe principle, the main axis passing down 
the third toe, and in the ox, on the two-toe principle, the 
main axis passing down between the third and fourth toe. 
(2) The formation in the ox of bony frontal organs (horns 
and horn-cores) for defence and their absence in the horse. 
(3) The different method of feeding involving a more com- 
plex stomach and loss of upper incisors in the ox. (4) The 
- different principle upon which the complex molars are 
evolved. (5) Other peculiarities, such as the presence of 
alisphenoid canal, of twenty-three dorso-lumbar vertebre, 
and of broad nasals in the horse. (1), (4) and (5) are 
characters of subordinal value, as they are distinctive of the 
Artiodactyla and Perissodactyla. 
Before leaving these two important types we may inquire—How do 
we know that they have been descended in the past from a common 
