OSTEOLOGY OF THE ARMORED DINOSATJRIA. 



59 



The anterior caudals are the largest in the whole vertebral column, and have 

 exceptionally high, backwardly directed neural spines, the summits of which terminate 

 in heavy, transversely expanded, rugose heads, as shown in figures 25 and 26. On 

 the anterior third of the tail these processes rapidly decrease in height posteriorly. 

 In specimen No. 4934 the sixteenth is only a little more than half the height 

 of the first caudal. Posteriorly the spinous processes continue to decrease in 

 height and disappear altogether on the thirty-seventh or thirty-eighth vertebrae. 



The transversely expanded summits become distinctly bifurcate in the mid- 

 caudal region, a condition prevailing as far back as the twenty-fifth or twenty-sixth 

 caudal. Posterior to this point the transverse expansion of the spine becomes 

 gradually less and less until the last few vertebrae have a spine that consists of a thin, 

 gradually shortening plate of bone 

 without any expansion of the summit. 

 These also differ from those more an- 

 teriorly in that they do not overhang 

 the posterior ends of the centra (pis. 15 

 and 16). The remaining caudals have 

 the neural canal roofed over even to 

 the terminal one, as shown in plate 16. 



Excepting the first or caudo-sacral, 

 the nine succeeding vertebrae have ver- 

 tically elongated transverse processes 

 which may be best described as resem- 

 bling the "cleats" to which ropes are 

 made fast on board ship (figs. 25 and 

 26). On the second caudal they ap- 

 pear as a thin vertical plate, springing 

 directly from the superior lateral sur- 

 face of the centrum, the lower part 

 being produced into an irregularly 

 rounded process which extends out- 

 ward, downward, and somewhat for- 

 ward. Unlike the transverse of the caudo-sacral, it is not in contact with the 

 ilium, nor is this plate perforated as in some of the Sauropoda. The third caudal 

 bears the heaviest and longest (vertically) transverse process of any posterior to the 

 first, and in S. stenops it is also the first of the series to send a flattened projection 

 above the superior point of attachment to the centrum (see b, fig. 27). This type of 

 process decreases rapidly in size, proceeding posteriorly, and in the eleventh caudal, 

 in No. 4934, it has changed to a short, blunt, triangular process which becomes 

 smaller and smaller and finally disappears altogether on either the sevententh or 

 eighteenth vertebra. 



The vertebrae are joined by closely fitting zygapophyses of moderate size. On 

 the anterior half of the tail they overhang at both ends, but posteriorly, especially on 

 the distal portion. The anterior zygapophyses are gradually prolonged anteriorly. 



Fig. 27 — Anterior caudal vertebra of Stegosaurus un- 

 gulatus Marsh. Probably the tentii. J NAT. size. 

 a, Side view, b, Front vtew. c, Face for chevron; 

 n, neural canal; p, transverse process; s, neural 

 spine; z, anterior zygafophtsis; z', posterior zygapo- 

 piitsis. After Marsh. 



