OSTEOLOGY OF THE ARMORED DINOSAURIA. 79 



is a rugose roughening of the surface, as shown in figure 43. The superior border is 

 broadly concave antero-posteriorly. Near the proximal end it expands transversely 

 and on the outer side a rugose ridge is developed extending nearly across the bone 

 and forming the anterior border of the acetabulum, presenting above a triangular 

 elongated roughened surface which meets the pubic peduncle of the iliurn. Pos- 

 terior to this ridge, the pubis is continued backward as a broad process, with a con- 

 cave roughened external surface that forms the inner wall of the acetabulum, as in 

 Triceratops. On the ventral posterior border of this process is a small surface which 

 articulates with the antero-inferior process of the ischium. 



Beneath the rugose ridge described above the somewhat slender postpubis is 

 given off, extending downward and backward at an angle of something less than 

 45° to the longer axis of the prepubic portion. 



The shaft of this portion of the pubis is.fairly uniform in vertical width, but 

 the lower longutidinal half is transversely thickened, with a rounded ventral border, 

 while the upper half thins out to a sharp edge, which is slightly roughened. 



Marsh a shows the postpubis in S. stenops as exceeding the ischium in length 

 (see fig. 42), while in the pelvis of 

 S. ungulatus it is represented as fmd 3 

 being somewhat shorter. $ "** ^^f^""- -~- -' , 



I am unable to determine from / — aa -— - ■ ■,, , ■-- - - ^r5^ 



our material whether this is a con- J^ jt j. , 



stant difference or not. Probably ^0^ ,JJt 



it is not. The pubic foramen in (l s/ p 



Stegosaurus exists always as a ^*-s 



notch the nosterior border bein°" FlG- iS ~ LEFT puEIS ° F stegosaurus ungulatus marsh. ■& nat. 



| b size. Outer view, p, prepubis; p', postpubis. After Marsh. 



open toward the acetabulum. The 



pubes, as with the ischia, appears to have been in contact only on the median line 



at their distal extremities. 



In Stegosaurus the boundaries of the acetabulum are even more completely 

 inclosed by bone than in Triceratops, which Hatcher 2 considered as approaching 

 most nearly to the mammalia in this respect. 



Specimen No. 7420, a right pubis and the only complete element in the collec- 

 tion, gives the following measurements: Greatest distance from end of pubis to the 

 end of postpubis, in a straight fine, 850 mm. ; greatest length of prepubic portion, 

 460 mm.; same measurement of No. 4934, 435 mm.; greatest length of postpubis 

 from anterior edge of notch to distal end, 510 mm. 



Ischium (is.) . — The ischium is a comparatively short, flat, triangular element, 

 showing a concave surface only on the Y-shaped proximal end and gradually taper- 

 ing toward the distal extremity. The larger of the two articular faces on the 

 proximal end meets the ischiac peduncle of the ilium, while the smaller and 

 transversely compressed surface articulates with the pubis a little forward of the 

 center of the acetabulum, as shown in figure 42. 



Viewed from above, the transversely rounded border sweeps downward and 

 backward from the heavy articular surface, with a gentle curve for over half its 



' Dinosaurs of North America, 1896, pi. 48, figs. 2 and 3. 2 Mon. 49, U. S. Geol. Surv., 1907, p. 57. 



