MESOZOIC ERA. AGE OF REPTILES. 347 



footed (Theropoda), and some curious plate-covered rep- 

 tiles (Stegosauria) (Fig 301). The Ornithopoda and some 

 Theropoda walked almost wholly on their hind-legs in the 

 manner of birds. The size of some of these reptiles is 

 almost inconceivable. A thigh-bone of an Atlantosaur, 

 found by Marsh, was six or seven feet long, and a vertebra 

 of an Amphiccelias, found by Cope, was six feet high to 

 the top of the spinous process. The Atlantosaur has 

 been estimated to have been seventy to eighty feet long ! 

 In the same beds, as already stated, were found the 

 remains of a bird and of seventeen species of marsupials. 

 A figure of one of these is herewith given (Fig. 302). 



FIG. 302. Right lower jaw of Diplocynodon victor (after Marsh), outside 

 view twice natural size. 



Disturbances which closed the Jura-Trias Pe- 

 riod. One of the most important changes which oc- 

 curred at the close of this period was the formation of 

 the Sierra Nevada Range. Until that time the Pacific 

 shore-line was east of the Sierra, and the place of this 

 range was a marginal sea bottom receiving sediment. 

 These sediments finally yielded at the close of this period 

 and were folded and swelled up into this great range. 

 Subsequent erosion sculptured it into its present grand 

 forms. Coincidently with this change in the West, there 

 were on the Atlantic border outbursts of igneous matter 

 forming the trap ridges. In the interior region there 



