50 



THE CEEATOPSIA. 



its inferior border, while the centrum of the ninth is still more imperfect, and those of the 

 succeeding dorsals have entirely weathered away. The impressions of these centra with 

 fragments of the bone adhering are preserved in regular series in the matrix and the spines, 

 transverse processes, and neural arches interlocked by their zygapophyses are still intact in 

 the matrix, most of them in a perfect state of preservation, as shown in fig. 48. Posterior 

 to the fourteenth dorsal the hard sandstone concretion in which the skeleton was found 

 embedded was much fractured by surface weathering. Impressions of the first and second 

 sacral centra are also preserved, together with the impression of the transverse process of the 

 first sacral and a considerable portion of the strong parapophysis or sacral rib, given off from 

 the point of union between the second and third sacrals and abutting against the base of the 

 pubic process of the ilium. Most of the right ilium is represented either by the actual bone 

 or by impressions of it in the matrix and the proximal portion of the right pubis is in position. 

 In this skeleton we are for the first time able to fix with certainty the number of presacral 

 vertebras in this genus. 



THE FIRST DORSAL. 



The chief difference between the first 

 dorsal and the last cervical is noticeable in 

 the transverse process and the tubercular rib 

 facet. The transverse process in the first 

 dorsal is longer, not so flat, more triangular 

 in cross section, and is inclined more directly 

 upward, rising above the level of the zyga- 

 pophyses. The tubercular rib facet is much 

 larger and looks downward and outward in 

 conformity with the shape and position of the 

 first thoracic rib, instead of forward and 

 slightly outward as in the cervicals. The 

 capitular facet is situated at the base of the 

 transverse process and on the superior border 

 of the centrum, as in the last cervical. The 

 neural spine is somewhat broader antero- 

 posteriorly than in the last cervical. In fig. 

 51 there is shown a disarticulated anterior 

 dorsal of Triceratops prorsus. The short and 



broad centrum, stout and somewhat elevated transverse processes, deeply incised posterior 



zygapophyses, and low neural arch are conspicuous features. 



Fig. 51.— A, Anterior dorsal of Triceratops prorsus (No. 4842, U. S. 

 National Museum, second dorsal of mounted skeleton), side view; 

 B, anterior view of same, s, Neural spine; t, transverse process; 

 h, capitular rib facet; z, anterior zygapophyses; z', posterior zyg- 

 apophyses; 71, neural canal; a, anterior end. One-eighth natu- 

 ral size. After Marsh. 



THE SECOND DORSAL. 



This differs from the preceding vertebra chiefly in its more elevated and more robust 

 transverse process and the increased size of the tubercular rib facet, which, as in the preceding 

 vertebra, looks outward and downward. The capitular facet continues to occupy a position 

 on the superior border of the centrum at the base of the transverse process. There is a deep 

 cavity in the side of the centrum not seen in the succeeding dorsals and less emphasized in 

 the preceding. 



THE THIRD DORSAL. 



This differs from the second dorsal, especially in the altered position of the capitular 

 rib facet, which has now shifted from its position at the superior border of the centrum, as 

 seen in the posterior cervicals and dorsals 1 and 2, to a position high up on the side of the 

 neural arch and between the anterior and posterior zygapophyses. The tubercular facet also 

 is somewhat reduced in size and faces upward and outward instead of outward and downward, 



