424 REPTILES xv. 16- 



the head very small, with a lightly built skull. The nostrils lay on the 

 top of the head and in *Diplodocus formed a single opening. This 

 seems to indicate that the animals were aquatic or amphibious, as 

 would in any case be suspected from the very large size, making it 

 unlikely that the legs could bear the full weight. *Diplodocus and 

 *Brachiosaurns were over 80 ft long and the weight of the latter must 

 have been nearly 50 tons. However, the structure of the vertebral 

 column shows that much weight was carried on the legs, for the verte- 

 brae are strong, though hollowed in places. Footprints of the animals 

 have been found. One or more of the digits bore claws. The skull 

 became relatively short and broad, and among the many puzzling 

 features of these giant animals is the weakness of the jaws and small 

 size of the teeth, mostly crowded towards the front of the mouth. 

 These teeth would have served well enough for cropping, but there 

 are no teeth on the hind part of the jaws and no provision for grinding 

 the food. Animals of large size can only have been supported by this 

 feeble apparatus if some very nutritious food was readily available. 

 This perhaps agrees with the small size of the brain, which was 

 several times smaller than the lumbar enlargement of the cord. 



17. Order *Ornithischia 



The second main group of dinosaurs appeared later than the sauro- 

 pods and possessed a 4-radiate pelvis, with the pubis directed back- 

 wards and an extra pre-pubic bone pointing forwards. The teeth were 

 restricted to the hind part of the jaws, the front bearing a beak. At the 

 front end of the lower jaw there was an extra bone (predentary). 

 These were herbivorous forms and they appeared in the Jurassic and 

 achieved their maximum in the Cretaceous, by which time the sauro- 

 pods had become less common. The earliest of the ornithischians 

 were bipedal animals, included in a suborder Ornithopoda, from the 

 Jurassic and Cretaceous. These animals, such as *Iguanodon, were 

 built on the same general lines as the pseudosuchians, from which 

 they were presumably derived. The skull was heavily built and adapted 

 for a herbivorous diet, with powerful muscles attached to a coronoid 

 process of the lower jaw. The bipedalism was less marked than in 

 saurischians and the fore-limbs less reduced. Several separate lines 

 then reverted to a quadrupedal habit. The trachodonts (*Hadro- 

 saurus) were a very successful group of amphibious forms in the 

 Cretaceous, with webbed feet. The teeth were suited for grinding, 

 parallel rows being present, making as many as 2,000 teeth in one 

 animal. In several types of hadrosaur the top of the head was pro- 



