58 



THE CEEATOPSIA. 



Proximally it is expanded and presents a rugosity for articulation with the ischiac peduncle, 

 while sending downward and forward a process which articulates at its extremity with the 

 pubis. The proximal end of the ischium forms the inferior and most of the posterior border 

 of the acetabulum. 



Fig. 63. — Posterior view of ischia of Triceratops prorsus 

 Marsh, No. 1822, Yale Museum, vb Pubic surface; 

 il, iliac surface. 



THE SHOULDER GIRDLE. 



it 



So far as at present known the scapula and coracoid 

 were the only elements preserved in the pectoral arch. 

 Nothing representing either a clavicle or a sternal has 

 yet been found. It is more than probable, however, 

 that sternals were present. 



THE SCAPULA. 



As shown in fig. 64, the scapula is long and flat, 

 especially at the upper extremity, which is thin and 

 somewhat expanded antero-posteriorly. Just below the 

 upper end of the scapula a ridge appears at about the 

 middle of its outer surface and continues downward 

 throughout about two-thirds of the length of the bone, 

 gradually approaching the posterior border of the blade 

 of the scapula, when it rapidly becomes more elevated 

 and is directed more abruptly backward, extending 

 beyond the posterior border of the blade of the scapula 

 and giving support to the glenoid cavity. This is doubt- 

 less homologous with the spine of the scapula in mam- 

 mals, and, like that element, it divides the blade of 

 the scapula into a smaller prescapula and a larger postscapula, with prescapular and post- 

 scapular fossae, as in the Mammalia. The scapula is broadest and thickest at the superior 

 border of the glenoid cavit}^, which is rather high and of moderate depth and is formed by 

 both the scapula and the coracoid, the former contributing rather more than the latter to its 

 construction. 



Fig. 64. — External view of right scapula and coracoid of 

 Triceratops prorsus Marsh, No. 4800, U. S. National 

 Museum, in mounted skeleton, sc, Scapula; g, glenoid 

 cavity; cr, coracoid; s, suture between scapula and 

 coracoid. One-eighth natural size. After Marsh. 



