O. C. Marsh — Skull and Armor of Stegosaurus. 415 



The Dermal Armor. 



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. Subse- 

 quent 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 por- 

 tions 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 pro- 

 tected by small, rounded and flattened ossicles having a regular 

 arrangement in the thick skin. One of these ossicles is shown 

 in Plate YII, figure 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 Plate YII, figure 

 3. Some of the species, at least, had somewhat similar plates 

 below the base of the tail, and one of these bones is repre- 

 sented in figure 2 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 Plate YIII, figures 2 and 3. In some other 

 forms, there were three pairs, and in S. stenops, but two pairs 

 have been found. 



In one large species, which may be called Stegosaurus sulca- 

 tus, there is at present evidence of only one pair of spines. 

 These are the most massive of any yet found, and have two 

 deep grooves on the inner face, which distinguish them at 

 once from all others known. One of these grooved spines is 

 represented on plate YIII, figures 4, 5, and 6. 



The position of these caudal spines with reference to the tail 

 is indicated in the specimen figured on Plate IX, which shows 

 the vertebrae, spines, and plate as found. 



