92 



BULLETIN 89, UNITED STATES NATIONAL MUSEUM. 



FlG. 56. — TUBERCULAR SPINE OF STEGO- 

 SAURUS UNGULATUS MARSH. ,1,, NAT. 



size, a, Superior view; b, inferior 

 view; c. end view. After Marsh. 



None of the isolated dermal structures figured by Marsh have been recognized 

 with any National Museum specimen (fig. 56). 



Nuchal plates. — The plates borne upon the neck immediately posterior to the 

 skull are small, thin, vertically elongated, with roughened basal ends, without 

 material transverse expansion, except in very old individuals. In specimen No. 

 4934 there are five of these small elongated plates as shown in plate 14. These 



gradually increase in height from front to back. As 

 shown by No. 7615 (pi. 23, figs. 2 and 3), they are flat- 

 tened on one side, probably the external, and slightly 

 convex antero-posteriorly on the opposite side. The 

 depth of the insertion of these plates in the skin 

 appears to be indicated by an abrupt transverse 

 constriction some 40 mm. above the basal end, well 

 shown in figure 57, g. The character of the surface 

 of this portion is also changed, inasmuch as the vascular grooves of the upper sur- 

 faces end abruptly at the point of contraction. In other specimens this surface is 

 roughened with fine interrupted ridges of bone, their general trend being antero- 

 posteriorly. In some of the above plates there is a slight transverse beveling of 

 the lower end, and all agree in having 



one large foramen leading up into the 1 2 



interior of the bone. 



Dorsal plates. — The small elon- 

 gated plates are succeeded by larger 

 more or less oval plates, with trans- 

 versely expanded bases, of very short 

 fore and aft extent as compared with 

 the longitudinal diameter of the upper 

 portion, apparently a provision for 

 greater freedom of movement such as 

 would be required in the region of the 

 shoulders (pi. 23, figs. 4, 5, and 6). In 

 specimen No. 4934 there are four of 

 these oval plates with bases cleft 

 longitudinally. These increase rapidly 

 in size, as is shown in plate 14, Nos. 6 

 to 8. The bases of these plates are 

 somewhat asymmetrical, with exterior 

 a«d interior surfaces about equally 

 rugose. The asymmetry of the bases 

 would indicate that the plates were 

 inclined slightly outward from the perpendicular when held in the skin. As in 

 those plates on the anterior portion of the neck the external side is flattened, while 

 the internal is slightly convex antero-posteriorly. Other specimens, as No. 7584 

 (pi. 23, fig. 5), show similar plates to have transversely enlarged bases, but with- 

 out median cleft. 



Fig. 57.— (1) Side and (3) posterior views of nuchal plate 

 of Stegosaurus sulcatus Marsh. Type. Cat. no. 4937, 

 U.S.N. M. J NAT. size, a, Anterior border; g, groove show- 

 ing DEPTH OF INSERTION IN THE INTEGUMENT; p, POSTERIOR 

 BORDER. 



