440 



MESOZOIC ERA— AGE OF REPTILES. 



of them walked habitually on their hind-legs alone, in the manner of 

 birds. (3.) Like birds and some mammals, many Dinosaurs tread on 

 their toes (digitigrade) and not like reptiles on the whole foot (planti- 

 grade). (4.) Like birds, also, many — but not all — had only three func- 

 tional toes, and therefore made tridactyle tracks ; and even the number 

 of toe-joints follows the order of those of birds — i. e., there were three 

 in the inner toe, four in the middle, and five in the outer toe. (5.) 

 Still more curious is the resemblance to birds, in the structure of the 



Fig. 691.— A, Dromseus; B, Dinosaur; C, Crocodile: As, astragulus; Ca, calcaneum, 



ankle-joint. In reptiles — as also in mammals — the joint is between 

 the shank-bones and the tarsus ; in birds, the astragalus and calca- 

 neum are consolidated with the shank, and the motion is below these 

 bones of the tarsus. Some Dinosaurs are like birds in this regard. 

 Fig. 691, ABC, illustrates this point. (6). Many Dinosaurs pos- 

 sessed a clavicle — a bone found in all birds and many mammals, but no 

 living reptile. We shall very briefly describe only the most re- 

 markable. 



The Iguanodon was one of the best known as well as one of the 

 largest. It was a huge herbivorous Dinosaur, found in the Upper 

 Jurassic and Lower Cretaceous of Europe. It takes its name from the 

 resemblance of its teeth (Fig. 692) to those of an Iguana — a living 

 herbivorous reptile, about four or five feet long, although in other 

 respects there is little affinity. Until recently, only portions of the 

 skeleton were found ; but the enormous size of these indicated an ani- 

 mal at least thirty feet long, and several times the weight of an ele- 



