520 CHORD AT A. 



cubcid, 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 (i) 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 (i) 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 vertebrae, 

 and of broad nasals in the horse. (i), (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 



