502 



GEOLOGY OF THE DENVER BASIN. 



increased their size and weight. This covering is clearly indicated by the 

 vascular grooves and impressions which mark the surface of both plates 

 and spines, except their bases, which were evidently implanted in the thick 



skin. 



CAMPTOSAURUS. 



Another group of dinosaurs, the Ornithopoda, is well represented in 

 the Atlantosaurus beds, on both the eastern and western sides of the 

 Rocky Mountains. This suborder includes the smaller herbivorous forms, 

 which have many bird-like features. The genus Nanosaurus, already 



Fig. 5s.— Skull of Camptosaurus medius Marsh; seen from the left side. One-fourth natural size, 

 n, anterior ii.iii.il opening; an, angular bone; h". hasioccipital; d. dentary; ./.frontal;/ jugal; 

 ;/. nasal: o. orbit; pd, predentary; pm, premaxillary ; y. quadrate: 8, squamosal; c a, surangular. 



Fn.. 59.— Fifth lower tooth of Camptosaurus medius. Natural size. 

 11, outer view; h. posterior end view; c, inner \ieu. 



described, is the smallest member of the group, or, in fact, of all the dino- 

 saurs, while Camptosaurus includes some species of quite large dimensions. 

 All this group are bipedal in locomotion, and thus quite unlike the gigantic 

 forms previously described from this horizon, which were all quadrupedal. 

 The general form and structure of the animals of this order are indicated 

 by the restoration on PI. XXIII, which represents the skeleton of one of the 

 typical species of the genus Camptosaurus. In tig. 58, above, is shown 



