EVOLUTION, VARIATION AND HEREDITY 479 



but, in spite of its greater length, Diplodocus was not so huge an 

 animal as Brachiosaurus, for while the former weighed about 30 tons, 

 the latter was over 40, and Brontosaurus was intermediate between 

 them. They were all apparently slow-moving swamp-dwellers, 

 walking on all fours. They had long necks and tails, and very small 

 heads compared with the bulk of the body. The carnivorous types 

 culminated in Tyrjnnosaurus, which reached a length of 47 feet. 

 It ran on its strong hind legs, and in this position its height was 

 from 18-20 feet. Its head is about 4 feet long, very massive, and 



FIG. 173. Restoration of Tyrannosaurus, based upon a specimen in the Ameri- 

 can Museum of Natural History. Length, 47 feet. Cretaceous, western 

 North America. After Lull, from Schuchert's " Historical Geology." 



provided with powerful teeth, so that altogether it was well fitted to 

 prey on the large herbivorous forms. 



The Ornithischia probably arose about the same time, or perhaps 

 a little later, than the Saurischia, and they show three distinct 

 lines of modification, and are remarkable not so much for their size 

 as for the extraordinary and often bizarre form assumed by their- 

 armour. The first line led to swift -running forms, which were bird- 

 footed and bipedal ; the second line produced forms with a great 

 development of armour in the form of plates and spines ; and the 

 third line is characterised by the development of horns and of a 

 large bony neck frill. The two last groups were quadrupedal. The 

 bird-like forms are represented by a number of animals like Iguanodon, 

 from the Cretaceous beds of Belgium and the Isle of Wight. This 

 was a large creature about 34 feet long with very powerful hind 

 limbs, and the hand bore a very well-marked spine-like thumb. 

 The most striking of the armoured Dinosaurs is perhaps Stegosaurus, 

 which reached a length of about 20 feet. Along each side of its 



