216 THE WOKLD OF LIEE 



feet long, and the whole head and body about nine feet, it must 

 have far exceeded in size the largest lion or tiger, and prob- 

 ably that of any carnivorous land mammal that has ever lived. 



In I^orth America these reptiles were also present in consid- 

 erable abundance. Some, forming the sub-order Theriodontia, 

 were allied to the Pariasauri, and were probably herbivorous; 

 while the Pariotrichida? were carnivores, as were also a very 

 distinct family, the Clepsydropidse. Of this latter group one 

 1 genus, Dimetrodon, is here figured as restored by Sir Ray 

 Lankester (Fig. 49). This is supposed to be allied to the liv- 

 ing Hatteria of New Zealand. These strange carnivorous rep- 

 tiles of this early period may have preyed upon numerous 

 herbivores which have not been preserved, as well as upon the 

 primitive insects and land Crustacea, which at this period were 

 probably abundant. 



The remarkable thing is, that some hundreds of species of 

 varied form and size, herbivorous and carnivorous, should have 

 been gradually developed, arrived at maturity, and completely 

 died out, during the comparatively short periods of the Permian 

 and Trias, or the interval between them. 



It is probable, however, that these transition periods really 

 occupied a very great length of time, since all known reptiles 

 seem to have originated during this era, though owing to unfa- 

 vourable circumstances the connecting links have rarely been 

 preserved. The singular Chelonia (turtles and tortoises) ap- 

 pear fully formed at the end of the Trias or in the earliest 

 Jurassic beds, as do the crocodiles, the aquatic Plesiosaurians 

 and Ichthyosaurians, the flying Pterodactyls, and the huge 

 Dinosaurs. All these have more or less obscure interrelations, 

 and their common ancestors cannot well be older than the 

 Permian, since the preceding Carboniferous offered highly 

 favourable conditions for the preservation of the remains of 

 such land animals had they existed. To bring about the modi- 

 fication of some primitive reptile or amphibian into all these 

 varied forms, and especially to bring about such radical changes 

 of structure as to develop truly aerial and truly oceanic rep- 



