192 DINOSAURS OP NORTH AMERICA. 



expanded, and in the specimen figured is firmly coossified with the cal- 

 caneiim. The two coalesce with the tibia and astragalus, and form a 

 smooth convex articulation for the ankle joint. There were only three 

 functional digits in the hind feet, as shown on PI. XLVIII, fig. 2. 



THE DERMAL ARMOR. 



The most remarkable feature about Stegosaurus is the series of ossi- 

 fications which formed its offensive and defensive armor. These consist 

 of numerous spines, some of great size and power, and many bony 

 plates of various sizes and shapes, well fitted for protecting the animal 

 against assaults. Some of these plates are a meter, or more than 3 feet, 

 in diameter. 



The spines were of different forms and varied much in size. Four 

 of these are represented on PI. L. All of those preserved are unsytn- 

 metrieal, and most of them are in pairs. One of the largest is shown 

 in fig. 2, which gives the more usual form and proportions. This speci- 

 men is over 2 feet in length. 



The osseous dermal covering of the Stegosauria was first described 

 by the writer from specimens found associated with several skeletons, 

 but not in place, and hence the position of the various parts was a 

 matter of considerable doubt. Subsequent discoveries have shown the 

 general arrangement of the plates, spines, and ossicles, and it is now 

 evident that, while all the group were apparently well protected by 

 offensive and defensive armor, the various species, and perhaps the 

 sexes, differed more or less in the form, size, and number of portions of 

 their dermal covering. This was especially true of the spines, which 

 are quite characteristic in some members of the group, if not in all. 



The skull was evidently covered above with a comparatively soft 

 integument. The throat and neck below were well protected by small, 

 rounded and flattened ossicles having a regular arrangement in the 

 thick skin. One of these ossicles is shown in PI. XLIX, fig. 1. The 

 upper portion of the neck, back of the skull, was protected by plates, 

 arranged in pairs on either side. These plates increased in size farther 

 back, and thus the trunk was shielded from injury. From the pelvic 

 region backward a series of huge plates stood upright along the median 

 line, gradually diminishing in size to about the middle of the tail. One 

 of these is shown in PI. XLIX, fig. 4. Some of the species, at least, had 

 somewhat similar plates below the base of the tail, and one of these 

 bones is represented in fig. 3 of the same plate. 



The offensive weapons of this group were a series of huge spines 

 arranged in pairs along the top of the distal portion of the tail, which 

 was elongate and flexible, thus giving effective service to the spines, 

 as in the genus Myliobatis. 



In Stegosaurus ungulatus there were four pairs of these spines, 

 diminishing in size backward. Two of the larger of these are shown 

 on PI. L, figs. 2 and 3. In some other forms there were three pairs, 

 and in S. sttnops but two pairs have been fouud. 



