ART10DACTYLA 277 



of the astragalus divided into two nearly equal facets, one for the 

 navicular and the other for the cuboid bone. The calcaneum with 

 an articular facet for the lower end of the fibula. Stomach almost 

 always more or less complex. Colon convoluted. Caecum small. 

 Placenta diffused or cotyledonary. Mammae 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. 

 Modification 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. Eeduction of the ulna from a complete 

 and distinct bone to a comparatively rudimentary state, in which it 

 coalesces more or less firmly with the radius. Eeduction 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 navicular 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-cylinder. 



4. Development of horns or antlers on the frontal bones, and 

 gradual complication of form of antlers. 



5. By inference only, increasing complication of stomach with 

 ruminating function 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 proba- 

 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, 



