544 



HISTORICAL GEOLOGY 



Unarmored Bipedal Herbivorous Dinosaurs (Unarmored Pre- 

 dentata). — The dinosaurs of this group were similar in general ap- 

 pearance to the carnivores, but differed in their less graceful build. 

 Some of them attained a large size, being as much as 30 feet in length 

 and standing 15 feet high (Iguanodon and Trachodon). The hind 

 legs of some were twice as long as the fore. The heads varied con- 

 siderably in difFerent 

 genera, being long and 

 rather slender in most, 

 but flat and ducklike in 

 one specialized form, the 

 duck-billed dinosaur 

 (Trachodon, Figs. 507, 

 508) . They were all alike 

 in having the front of the 

 jaw toothless and covered 

 with horn. The rear 

 portion of the jaws, how- 

 ever, was in one genus 

 (Trachodon) provided 

 with a battery of chop- 

 ping and shearing teeth, 

 composed of 45 to 60 

 vertical and 10 to 14 

 horizontal rows (Fig. 

 509), though the rows 

 were not all in use at the 

 same time, the total num- 

 ber of teeth in some indi- 

 viduals being more than 

 2000. Others (Iguano- 

 don and Camptosaurus) 

 had only one row of shearing teeth in use at one time. The teeth 

 were replaced as rapidly as they were worn out. Teeth of this 

 sort indicate that their possessors chopped or sheared their food 

 and were able to live on tough, hard vegetation, such as the 

 cycads and, perhaps, even the siliceous horsetails of the period. 



They had three toes on the hind feet, terminating in hoofs (Tra- 

 chodon), or claws (Camptosaurus). The fore limbs had three well- 

 developed fingers, with one, or sometimes two other rudimentary 



Fig. 507. — Skeleton of the herbivorous, duck- 

 billed dinosaur, Trachodon. (After American 

 Museum of Natural History.) 



