Lull, The Armored Dinosaur Stegosaurus ungulatus etc. 



675 



the teeth are small, weak structures with crenulated margins 

 and entirely invisible from the outside of the skull and jaws, 

 implying a^food of a very yielding character which did not re- 

 quire the forcible mastication of that of Iguanodon or especially 

 of the late Cretaceous Trachodon in which the powerful dental 

 battery of 2,000 teeth reaches its highest perfection. 



III. The Armamen t. 



The exoskeleton of Stegosaurus included five types of struc- 



ture, of which the first were small, rounded ossicles found in one 



instance in position beneath the throat. The known armor of 



the back consisted of four distinct forms of plate in each of which 



Fig. 



Brain and sacral nerve mass. 



the dorso-ventral diameter is enormously developed and often the 

 antero-posterior one as well instead of a normal horizontal ex- 

 panse, the result being the production of huge upstanding plates 

 arranged in pairs on either side of the median line of the neck, 

 trunk and proximal part of the tail, and immense bony spines, 

 possibly varying in numbers with the different species, borne on 

 its distal end. The plates are differentiated in that those borne 

 upon the neck are short in fore and aft diameter but with a heavy 

 base divided longitudinally by a cleft which was borne astride 

 the transverse processes and cervical ribs. The trunk plates in- 

 crease rapidly in dimensions to a maximum size of 71 X 66 cm. 

 ( = 28 x 26 inches) and a weight of 17,7 kilogrammes, about 

 40 pounds for the pair borne over the pelvis. Here the base was 

 thick and rugose but without the longitudinal cleft. These plates 

 were borne above the transverse processes and the proximal 

 portion of the ribs until the sacrum was reached with its widely 

 expanded neural spines to which the weight was transferred, 

 the two rows of plates approaching each other more nearly than 

 in the more anterior region. 



This same type of plate, though with a base somewhat shor- 

 tened and at the same time much broader, was borne over similar 



43* 



