OSTEOLOGY OP THE ARMORED DINOSAURIA. 



95 



the vertebral centra," for surely were they so attached the function of these muscles 

 would be destroyed. Like all the other dermal structures of this animal, I believe 

 these bases were embedded entirely within the thick skin. 



Dermal spines. — Of the remaining dermal structures only the spike-like spines, 

 situated near the end of the tail, are yet to be considered. The number of these 

 spines, with the possible exception of S. ungulatus, appears not to have exceeded 

 four, arranged in two pairs. Lucas was the first to reach this conclusion from 

 the evidence of two specimens which showed them in place (pis. 15 and 16). 



That four is the correct number is now substantiated by the evidence shown 

 by six individuals, five of which are A 

 in the National Museum. The fore- 

 most pair is always the larger of the 

 two. I have before me 30 of these 

 spines, representing at least 10 individ- 

 uals. These specimens exhibit a con- 

 siderable variety of shapes and sizes, 

 ranging from 7 inches (175 mm.) long 

 in a young individual (pi. 25, fig. 2) 

 to more than 3 feet (985 mm.) in the 

 type of S. longispinus, an adult speci- 

 men (fig. 60, .4). All agree in having 

 oblique bases, but in the expansion of 

 this end they vary from the nonex- 

 panded ends of S. longispinus and S. 

 stenops to the exceedingly large bases 

 of S. sulcatus (fig. 60). The spines of 

 the young individuals (pi. 25, figs. 1 

 and 2) are without rugose roughening 

 of the bases, but the adults are very 

 rough, reaching their maximum de- 

 velopment in S. sulcatus (pi. 18). 



All of the spines have the surface 

 of the oblique end shallowly concave 

 dors o-ventr ally. On this end there is 

 always one large pit, which leads up 

 into the interior of the shaft. (See 

 /, fig. 65.) It probably transmitted a blood vessel supplying nourishment to 

 cells, which, as shown by various cross-sections, is very cancellous, implying a bone 

 of light weight. The spines gradually taper, except in S. longispinus, from the 

 expanded bases to the subacute apex, the lower two-thirds being characterized by 

 sharp edges fore and aft; but in many of the specimens the upper third is 

 subcircular in cross-section (fig. 59, c). Like the flat plates, the external surfaces 

 are covered by vascular impressions (pis. 16 and 17). 



The anterior pair in all instances give evidence of having been more deeply 

 embedded in the skin than the posterior pah. That these spines were often 

 broken or injured in life is shown by three specimens, two of which have the top 



Fig. 60.— Dermal tail spines. A, Stegosaukus longispinus. 

 Type. D 54, University of Wyoming collection. B, 

 Stegosaukus sulcatus. Type. Cat. no. 4937, U.S.N.M. 

 C, Stegosaukus ungulatus. Cat. no. 6099, U.S.N.M. B, 

 Stegosaueus stenops. Type. Cat. no. 4934, U.S.N.M. 

 All figures & natural size. 



