THE PELVIS. 



57 



Ceratopsia. The external and internal borders each describe a sigmoid curve. The anterior 

 end of the ilium is broad, the posterior narrow. The width of the ilium continues uniform 

 from the anterior extremity to a point just back of the ischiac peduncle. 



THE PUBIS. 



As shown in fig. 62, the pubis in Triceratops is composed of a well-developed prepubic ele- 

 ment and a rudimentary postpubis. The shaft is constricted medially and rather flat, the 

 vertical diameter being the longer. It expands distally into a rather broad blade and is directed 

 downward and forward, but not inward. The distal extremities of the pubis were not in con- 

 tact. Near the proximal end of the pubes the external surface is produced into a projecting 

 rugose ridge, which extends entirely across the bone and forms the anterior border of the 



Fig. 62.— Left pubis ol Triceratops prorsus Marsh, No. 4842, U. S. National Museum, in mounted skeleton; B superior view; C, external 

 view, a, Proximal end; b, surface lor pubic peduncle; c, postpubis, broken away in A and C; d, distal end; e, surface lor contact with 

 ischium. One-eighth natural size. After Marsh. 



acetabulum, presenting above an articular surface for contact with the pubic peduncle of the 

 ilium, and beneath another for contact with the anteroinferior projection of the proximal end 

 of the ischium. Posterior to this ridge the pubis is developed into a broad process, which is 

 rugose on its external surface and forms the internal wall of the acetabulum, which is more 

 nearly closed in the Ceratopsia than in any of the other herbivorous dinosaurs. In this respect 

 these dinosaurs approach those conditions which prevail in the Mammalia. 



THE ISCHIUM. 



This is the most slender of the bones of the pelvis. As shown in figs. 60 and 63, it con- 

 sists distally of a slender, rod-like shaft, which curves downward and inward and meets that 

 of the opposite side medially in an extended cartilaginous symphysis, as shown in fig. 63. 



