78 



BULLETIN 89, UNITED STATES NATIONAL MUSEUM. 



pelvis which overhangs the posterior ribs. Viewed from above the superior crest 

 curves inward and is closely and firmly united with the neural arches of the sacrum. 

 The ilium does not roof over the cavities between the transverse processes as 

 described by Marsh 1 but in adult individuals these cavities are completely covered 

 over, as shown in the sacrum of S. stenops, No. 4934, by the coalescence of the 

 antero-posterior expansion of the superior surfaces of the transverse processes. 

 Beginning just anterior to the pubic peduncle the superior surface widens rapidly 

 in a transverse direction posteriorly, particularly on the outer side where the upper 

 portion is produced outward as a heavy shelf of bone with a thickened rounded 

 outer border overhanging the ischiac peduncle. Posterior to the underlying ischiac 

 articulation the iliuni contracts rapidly in a transverse direction to a short, thickened, 

 truncate end placed on the inner side of the bone with roughened surfaces, which 

 has a greater vertical than transverse diameter. The greatest vertical thickness 

 of the ilium is through the pubic peduncle and from this point posteriorly the 

 bone is much depressed. 



Fig. 42.— Pelvis of Stegosaurus stenops Marsh. t v Nat. size. Side view, a, Acetabulum; il, ilium; 

 is, ischium; p, pubis; p', postpubis. After Marsh. 



The acetabular portion of the ilium is large and shallow. The surface for 

 articulation with the ischium, while not prominent, is nevertheless of good size, 

 consisting of a roughened transversely elongated triangular area with the point of 

 the triangle directed inward. 



The pubic peduncle while not extending far below the acetabular surface is 

 broad and heavy as in Triceratops. 



On the internal side above the acetabulum the surface is deeply concave verti- 

 cally with cupped depressions which articulate with the heavy expanded ends of 

 the sacral ribs. 



Pubis (p.) — The pubis in Stegosaurus is composed of a flattened, somewhat spatu- 

 late prepubic portion, and a slender, more elongated postpubis. (See fig. 43, p and 

 p'.) The anterior portion is a thin, vertical blade of bone, terminated usually by a 

 vertically rounded anterior end, although a few specimens show it as being cut off 

 squarely. While this end is not especially thickened, in fully adult specimens there 



' Amer. Journ. Sci., vol. 21, 1881, p. 168. 



