uabsh.i STEGOSAUKUS. 189 



THE SACRUM. 



The true sacrum of Stegosaurus is composed of four well-coossified 

 vertebrae. In fully adult animals the pelvic arch may be strengthened 

 by the addition of one or more lumbar vertebras, as in the specimen 

 figured in PI. XLVI, fig. 5, where two are firmly cousolidated with the 

 sacrum. The centra of the sacral vertebra? are solid, like the others in 

 the column. Their neural arches are especially massive, and the spines 

 have high and expanded summits. The transverse processes of the 

 sacral vertebrae are stout vertical plates, which curve downward below 

 and unite to meet the ilia. Each vertebra supports its own process, 

 although there is a tendency to overlap in front. There is a gradual 

 increase in size from the first to the last sacral vertebra, and the first 

 caudal is larger than the last sacral. The neural cavity of the sacrum 

 is described below. 



THE CAUDAL VERTEBR.E. 



The caudal vertebras present the greatest diversity, both in size and 

 form. The anterior caudals are the largest in the whole vertebral 

 series, and are highly modified to support a portion of the massive der- 

 mal armor. The articular faces of their centra are nearly plane and 

 very rugose. The neural spine has an enormous development, and its 

 summit is expanded into a bifurcate rugose head. These caudals are 

 very short, and their neural spines nearly or quite in apposition above. 

 Their centra have no distinct faces for chevrons. The transverse proc- 

 esses are expanded vertically, and their extremities curve downward. 

 Farther back the same general characters are retained, but the centra 

 are more deeply cupped and the spines less massive. Figs. 5, 6, and 7 of 

 PI. XLY show anterior caudal vertebras. The chevrons here have their 

 articular ends separate and rest upon two vertebras, as shown on the 

 same plate. 



In the median caudals, figs. 8 and 9, the spine has greatly diminished 

 in height, and the faces for chevrons are placed on prominent tubercles 

 on thepo^tero-inferior surface. The lower margin of the front articular 

 face is sharp and the chevrons do not meet it. In the more distal 

 caudals the neural spine and zygapophyses are reduced to mere rem- 

 nants, but the chevron facets remain distinct. These vertebras, as well 

 as those farther back, have their centra much compressed. The caudal 

 vertebras are remarkably uniform in length throughout most of the 

 series. 



THE SACRAL CAVITY. 



During an investigation of one skeleton of this genus the writer 

 found a very large chamber in the sacrum, formed by an enlargement 

 of the spinal canal. This chamber was ovate in form, and strongly 

 resembled the brain case in the skull, although very much larger, 

 being at least twenty times the size of the cavity which contained the 

 brain. This remarkable feature led to the examination of the sacra of 

 several other individuals of Stegosaurus, and it was found that all had 



