IV.] REPTILIAN EVIDENCE. 151 



epochs a single mammalian fauna was spread over Europe and 

 North America. This being so, it is a fair inference that a similar 

 fauna characterised a considerable portion of Asia ; while the 

 occurrence of the above-mentioned genus Tritylodon points to the 

 conclusion that it likewise ranged over Africa. Accordingly, it 

 would appear that not only did the whole of Arctogaea then form 

 a single zoological realm, but that this realm was indivisible into 

 regions. 



The evidence for this unity is, however, by no means restricted 

 to mammals, but is supplemented and extended by vid 

 the extinct reptiles of the Secondary epoch of the Secondary 

 earth's history. During the Triassic and early 

 Jurassic periods there flourished an extinct ordinal group of rep- 

 tiles known as the Anomodontia, remarkable for many structural 

 resemblances to mammals, and likewise for the peculiarities of 

 their dentition. As a well-known example of one section of this 

 group may be cited the dicynodonts, in which the teeth were 

 reduced, at most, to a single pair of tusks in the upper jaw, the 

 remainder of the jaws being ensheathed in horn to form a beak ; 

 whereas Galesaurus represents a second section in which the teeth 

 simulate those of the carnivorous mammals. These anomodonts 

 are known to have been spread over Europe, India, Africa, and 

 North America ; the dicynodont types from the three areas first 

 named being so alike that there is little question that some of 

 them were generically identical. The North American forms, 

 which mostly or exclusively come from beds assigned to the 

 Permian epoch, do not include dicynodonts, but are allied to 

 certain other African families, and are also closely related to their 

 European contemporaries. 



If we turn to another order of the same class, namely the 

 Dinosauria, as represented by the Iguanodon of Europe and the 

 Atlantosaurus of the United States, we find not less well- 

 marked similarities in the Jurassic and Cretaceous fauna of the 

 whole of Arctogaea, this group being represented by closely allied, 

 and in many cases generically identical forms, not only in Europe, 

 India, and South Africa, but likewise in Madagascar and South 

 America. For instance, in that section of the order known as 

 the Sauropoda, which includes the most gigantic forms, and is 



