54^ 



HISTORICAL GEOLOGY 



one family is Stegosaurus (Greek, siegos, roofed, and saurus, reptile), 

 an animal of greater bulk than an elephant. The restoration (Figs. 

 511, 512) shows two rows of broad plates on either side of the back- 

 bone, varying from a few inches to two feet in height and less than an 

 inch in thickness, except where they were embedded in the skin, and 

 with spines near the end of the tail six inches to over three feet in 

 length. The stout fore limbs are much smaller than the hind, but a 



study of the joints shows that the 

 creatures were quadrupeds. The 

 front of the jaw was toothless and 

 covered with horn as in the preced- 

 ing group. The teeth in the back 

 of the mouth were weak shearing 

 teeth, not strong enough to masti- 

 cate the coarser vegetation of the 

 time ; and they must, therefore, 

 have fed, for the most part, on 

 succulent plants. It is possible 

 that they lived on the land bor- 

 dering marshes. One of the most 

 remarkable features of this unique 

 reptile is to be seen in its nervous 

 system. The brain is estimated 

 to have weighed only about two 

 and a half ounces (about one- 

 fiftieth that of an elephant of 

 smaller size), while the enlarge- 

 ment of the spinal cord above the 

 hips is twenty times larger than 

 the brain. This "hind brain" 

 was probably the nervous center for the great muscles of the tail. 

 It is likely that Stegosaurus did not face its enemy, but protected 

 itself by swinging its long, powerful tail, which, however, was not 

 very flexible. The long hind limbs suggest the possibility of con- 

 siderable speed, and the fore limbs are so constructed as to make it 

 possible for the animal to pivot the body rapidly so as to keep the 

 tail to the enemy. These reptiles were descended from unarmored, 

 herbivorous dinosaurs with a bipedal habit. 



Two causes of the extinction of this family are suggested : the change 

 in the vegetation to modern plants (p. 568), and the senility of the race, 



Fig. 510. — The skin of Trachodon. 

 The illustration is from a photograph 

 of the impression made by the skin on 

 the sediments in which it was buried. 

 (After Professor Osborn.) 



