JR. S. Lull — Dinosaurian Distribution. 13 



rus, however, has relatively feeble teeth, and must have fed 

 upon the most succulent of terrestrial plants. The Ceratopsia, 

 living as they did toward the close of the Upper Cretaceous, 

 were surrounded by a virtually modernized flora, and hence 

 may have had feeding habits very similar to those of the sub- 

 tropical browsing ungulates of to-day. 



Trachodon, however, presents more of a problem on account 

 of its undoubted aquatic habits. Here the anterior, toothless 

 part of the mouth in the most highly specialized types became 

 broadened and depressed into a duck-like form, and, while 

 undoubtedly sheathed with a horny or leathery integument, 

 probably did not have the shearing mechanism so perfectly 

 developed as in the true terrestrial plant-feeders. On the 

 other hand, the dental battery reaches its greatest perfection 

 in Trachodon, consisting as it does of " from 45 to 60 vertical 

 and from 10 to 14' horizontal rows of teeth, so that there were 

 more than 2,000 teeth altogether in both jaws." (Brown, 

 1908, p. 53.) The immense number, especially of those in 

 reserve, implies a very rapid wear and consequent replacement 

 of the teeth ; which, together with the sharp, serrated, shear- 

 ing edge which the collective teeth of a jaw present, argues 

 strongly in favor of the idea as expressed by Brown (loc. cit., 

 p. 55) that some species of Equesetse, the remains of which are 

 the most abundant among the plant relics entombed with these 

 dinosaurs, supplied them with food. The broad duck-like 

 muzzle would be admirable for dislodging the rhizomes from 

 their resting place, while the abundance of silica in the cuticle 

 of the plant would necessitate just such a dental battery as the 

 Trachodonts possessed for its proper mastication. 



IV. Geological Distribution. 



Both geologically and geographically the dinosaurs show a 

 peculiar discontinuous distribution, due in large measure to the 

 imperfection of our records, but also to the fact that they were 

 principally terrestrial types and that the preservation of their 

 remains in water-laid rocks is largely the result of accident. 



Theropoda. 

 Geologically the Theropoda have the greatest range, as they 

 are first found in the Lower Muschelkalk of Europe and con- 

 tinue on until the end of the Mesozoic. 



Triassic. 



Of the Triassic forms, a very complete series is found in 



central Europe, of which one of the most primitive genera is 



Thecodontosaurus, which also had the widest distribution both 



geographically and in time (see fig 2). From the Thecodonts)- 



