74 HUMAN BIOLOGY 



reptiles so nearly approached the mammalian grade in so 

 many characters of their dentition, jaws, skull, backbone 

 and hmbs that they almost deserved to be called mammals. 

 In the uppermost Triassic and later ages the Multituber- 

 culates flourished. These were peculiarly specialized rodent- 

 like forms, probably independently derived from the cyno- 

 dont grade and certainly not in line with the higher mammals. 

 Then in the Jurassic period there were various members 

 of at least seven different families of small mammals repre- 

 senting early experiments along mammalian Hues. Most 

 of these famihes left no recognizable or known descendants 

 in later ages, but in one of them, including several famous 

 fossil jaws from the Lower Jurassic of Oxford, England, to 

 which the name Amphitherium was applied, the lower 

 molar teeth distinctly foreshadow the "tuberculo-sectorial" 

 type. This was characteristic of the earliest placental mam- 

 mals of the Age of Mammals, and the whole science of 

 odontology or evolutionary study of the teeth (Osborn, 1907; 

 Gregory, 1922) leads us to predict the discovery of tuberculo- 

 sectorial lower molars in the Jurassic forerunners of the 

 placental stock, of which the order of Primates was a later 

 outgrowth. 



Only a single humerus and a single femur belonging to 

 these far-off Jurassic mammals are sufficiently well known 

 to have been closely studied but even although it is not 

 clear as to which kind of contemporary jaws and teeth 

 they belong with, yet again they are of great value; for, as 

 recently shown by Dr. G. G. Simpson, the precise arrange- 

 ment of their various parts and processes, in the light of 

 what is known of the relations of bones and muscles among 

 recent reptiles, monotremes and typical mammals, shows 

 that these limb bones of Jurassic mammals were intermediate 

 in details between the cynodont type below and the typical 

 mammalian grade above. In other words, these Jurassic 

 mammals were raising their bodies further from the ground 

 and preparing the locomotor apparatus for its next great 

 conquest, the invasion of the trees. It is not without sig- 

 nificance also that in the Cretaceous period preceding the 

 great expansion of the mammals at the opening of the Age 

 of Mammals (Osborn, 19 10), the dominant type, so far as 



