OSTEOLOGY OF THE ARMORED DINOSAURIA. 



99 



flesh. On the other hand, the base is extremely rugose and the only part adapted 

 for insertion in the skin or for the attachment of other connective tissues. Further- 

 more, specimen No. 4934 shows (pi. 2) the plates above the pelvic region retained 

 in the rock in such an erect posture, while those more anteriorly are folded back 

 underneath the skeleton, a thing manifestly impossible in plates naturally pro- 

 cumbent on either side. 



The rugose character of the bases of the plates, as well as of the spines, implies 

 an extremely thick skin in which these were embedded. The ossicles, plates, and 

 spines are all strictly dermal structures, for in no instance is there indication of 

 articulation with subadjacent bones. The vasicular grooves and impressions which 

 mark the surfaces of both plates and spines indicate that they were inclosed by a 

 horny covering which in life would 

 somewhat increase their size, as shown 

 in figures 61, 62, 63, and 64. 



The primary function of this armor 

 must have been that of defense, not, 

 perhaps, in the sense of use as actual 

 defensive instruments, but' protective 

 to the extent of giving the animal a 

 most formidable appearance. It is 

 true that the spines near the end of 

 the tail may have been of some use 

 as defensive weapons, but they could 

 have been of comparatively little value 



for offensive purposes, although it has been contended that this was their chief 

 function. To be effective in such a capacity, the tail should be flexible to a 

 considerable degree, and this does not appear to have been the case. In the 

 first place, the caudal vertebrae are joined by closely fitting zygapophyses, which 

 are present nearly to the distal end of the tail; moreover, the articular ends of 

 the centra are rather abruptly truncated, not rounded or beveled, as in those 

 animals having a flexible tail. These structural details, combined with the series 

 of plates along the dorsal side, must have made the tail of Stegosaurus a heavy, 

 stiff appendage, incapable of more than cumbersome lateral movements and wholly 

 unsuitedfor use on an active enemy. 



The facts relating to the dermal armor which now appear to be established 

 from this study of the Stegosaurian remains in the United States National Museum 

 are: 



(1) That the armor of the neck, back, and tail was formed by two rows of erect plates, the ele- 

 ments of one row alternating with those of the other. 



(2) That the total number of plates in the two rows was not less than 20 and not more than 22. 



(3) That the position of the largest plate of the series appears to be above the base of the tail and 

 not over the pelvis. 



(4) That the usual number of dermal spines on the tail is four, arranged in two pairs. 



Fig. 64. — Section of distal portion of the tail of Stego- 

 saurus. S, Deemal spine; V, vertebra. Modified from 



