OSTEOLOGY OF THE AEMOEED DINOSAUEIA. 79 



is a rugose roughening of the surface, as shown in figvixe 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 wliich meets the pubic peduncle of the ihum. 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 wliich 

 articulates with the antero-inferior process of the iscliium. 



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 tliis 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, wliich is sUghtly roughened. 



Marsh ^ 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 

 being somewhat shorter. 



I am unable to determine from 

 our material whether this is a con- 

 stant difference or not. Probably 

 it is not. The pubic foramen in 

 Stegosaurus exists always as a 

 notch, the posterior border being ^■«- «.-left pubis of stegosaukus ungulatus maksu. a nat. 



' ^ 1 mi ^"'^- OUTEK \nEW. p, PK^UBIS; p', POSTPUBIS. Aftek Maksii. 



open toward the acetabulum. The 



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



at their distal extremities. 



In Stegosaurus the boundaries of the acetabulum are even more completely 

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

 most nearly to the mammalia in tliis 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 Une, 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 iUum, wliile 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 romided border sweeps downward and 

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



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



