E. S. Lull — Armor of Stegosaurus. 209 



Fig. 10. 



Fig. 10. Section of the distal portion of the tail through the spines. 

 s, caudal spine ; other letters as in fig. 7. 



The four sections of Stegosaurus which I present will show 

 the relationship of the plates to the underlying skeletal 

 elements. The first section (fig. 7), that through the neck, 

 shows the plates with bifid base astride the transverse process 

 of the vertebra, and the second, that through the trunk (fig. 8), 

 the immense broad-based plates borne over the transverse 

 process and ribs. A beautiful mechanical device is shown in 

 that the transverse process is triangular and the rib T-shaped 

 in cross section in the armor-bearing region, giving the 

 maximum of strength, a wide bearing surface and a minimum 

 expenditure of material. The significance of the great eleva- 

 tion of the transverse process is also apparent. 



In the sacral and anterior caudal region the bases of the two 

 rows of plates are approximated, and now the summit of the 

 neural process broadens out to support their weight, as 

 indicated in the third section (fig. 9). This broad-topped type 

 of neural process ceases with the proximal third of the tail and 

 indicates the beginning of the flexible aggressive weapon of 

 offense bearing the sharp-edged spine-plates and caudal spines 

 which are inserted obliquely into the muscular mass on either 

 side in the angle formed between the neural process and the 

 centrum (fig. 10). 



Some of the larger spines, notably that described by Marsh 

 as the type of Stegosaurtis sulcatus, have the base divided by 

 an asymmetrically placed longitudinal ridge (fig. 11) into two 

 facets which seem to have borne against the neural process and 

 centrum of the vertebra. This character is only present in 

 very large spines which have a deep insertion into the under- 

 lying tissues. Ordinarily the insertion seems to be too shallow 

 to give rise to the facets. 



