52 



THE CEKATOPSIA. 



Seen from below, as shown in fig. 53, the sacrum, presents a strong median bar formed by 

 the united sacral centra, each of which gives rise on either side to a transverse process or sacral 

 rib. The extremities of these transverse processes form together the periphery of an oval, so 

 that the outline of the sacrum is an oval with the antero-posterior diameter much the longer. 



The inferior border of the sacrals is 

 marked by a broad, shallow groove com- 

 mencing with the centrum of the third 

 sacral and extending to that of the tenth. 

 The first sacral, or that vertebra which I 

 have interpreted as a dorso-sacral, sup- 

 ports a short, flat diapophysial process, 

 which curves backward at its distal end 

 and is firmly united with a similar but 

 somewhat longer process from the second 

 sacral. These two processes inclose be- 

 tween them an elongated foramen. The 

 diapophyses of the three succeeding verte- 

 brae are likewise united at their extremities 

 and inclose elongate foramina, as shown in 

 fig. 55, where a superior view of the same 

 sacrum is shown. Neither the first nor the 

 second sacral gives off parapophysial proc- 

 esses, and it might therefore be better to 

 regard both these vertebrae as dorso- 

 sacrals, as Marsh suggested. At the union 



of the second and third sacrals a very 

 strong parapophysis is given off. This 

 process arises quite as much from the sec- 

 ond sacral as from the third, and it is from 

 this fact that I have regarded this vertebra 

 as a true sacral rather than a dorso-sacral. 

 The unusually strong parapophysial proc- 

 ess springing from the union of the second 

 and third sacral centra is directed back- 

 ward and outward. It is much expanded 

 at its distal extremity, where it unites with 

 similar processes springing from the union 

 of the three succeeding sacral centra to 

 form the strong acetabular bar of the 

 sacrum constituting the superior and inner 

 wall of the acetabulum. This acetabular 

 bar is formed by the union of the distal ex- 

 tremities of the four anterior parapophyses, 

 which inclose a series of three large, elon- 

 gated foramina, larger than those inclosed 

 by the anterior diapophyses. Back of the 

 acetabulum the transverse processes are simple and decrease in length from the seventh to the 

 tenth sacral, the transverse process of the latter being greatly reduced in length. 



Only four of the sacrals bear diapophyses and parapophyses. These are the third, fourth, 

 fifth, and sixth, and it would perhaps be better to consider these four vertebrae as the only true 



FIG. 53.— Inferior view of sacrum of Tricemtops prorsus, No. 4842, U. S. 

 National Museum, in mounted skeleton, a, Anterior sacral: p, pos- 

 terior sacral; s, neural spine of last sacral; z, anterior zygapophyses 

 of first sacral; 1 to 10, transverse processes of left side. One-eighth 

 natural size. After Marsh. 



