716 MAMMALIA [CH. 



jaws, but in one case a scapula with a facette for a coracoid and an 

 interclavicle have been found which bear out the conclusions founded 

 on the jaws. At the same time other jaws have been found which 

 show teeth of a different kind. These have molar teeth of the 

 tritubercular pattern, but the angle of the jaw is inflected and these 

 have been referred to the Metatheria. As however the latter group 

 owe some of their peculiarities to degeneracy it would be better to 

 regard these jaws as remains of the direct forerunners of Eutheria 

 from which the Metatheria represent a side line. 



When we come to the sands and clays lying above the Chalk 

 which constitute the Tertiary "rocks," we find in many localities 

 a rich assemblage of remains of undoubted Mammalia of the 

 Eutherian type. The oldest horizon or Basal Eocene shows remains 

 of animals called Condylarthra and Creodonta. Both groups are 

 small plantigrade animals, with 44 teeth, but in the first group the 

 cusps of the tritubercular molars are blunt, and in the second sharp 

 and pointed. In this small distinction the beginning of the cleft is 

 seen which widens into the chasm now separating Ungulata and 

 Carnivora. Modern Insectivora are the little modified descendants 

 of the Creodonta, whilst the so-called " Sub-ungulata " may be 

 regarded as survivors of the Condylarthra which have undergone 

 modifications in their dentition. In the next horizon or Lower 

 Eocene traces of the Primates appear as Lemuroidea, the marks 

 discriminating them from Creodonta being the enlargement of the 

 orbit and its surrounding ring of bone, while the molar teeth have 

 a fourth tubercle. At the same time the Condylarthra begin to 

 show Horse-like forms (Phenacodus), still with five fingers and five 

 toes and of the Sub-ungulate type, but true Ungulata now appear 

 with the bones of wrist and ankle in transverse rows and a reduced 

 number of toes. The earliest of these, the Lophiodontidae, were 

 Perissodactyla, and iii the shape of the face some recall the Horse, 

 others the Rhinoceros, though the limbs were like those of Tapirs. 

 The cusps on the teeth were four in number, and were commencing 

 to coalesce into ridges. Rodentia also make their appearance as 

 Tillodontia, animals with one pair of large incisors in each jaw, but 

 with the other incisors and the canines present; these forms are 

 easily derivable from the Creodonta. The origin of the Artiodactyla 

 becomes apparent in the next horizon, the Middle Eocene, a host of 

 small Pig-like animals making their appearance which in higher 

 formations gradually differentiate themselves into the families of 

 Artiodactyla. The ancestors of the South American Edentata, which 

 at the previous horizon were not separable from Creodonta except 



