100 THE CERATOPSIA 



There is nothing distinctive about the vertebrae of Brachyceratops. The sacrum is of interest, 

 however, as it is the most immature ceratopsian sacrum on record. "The sacrum in the paratype, 

 No. 7953, is represented by all the centra, portions of the neural processes of the posterior vertebrae, 

 and numerous detached sacral ribs. . . . The centra are all suturally united and in an adult individual 

 would doubtless be firmly coossified, as in other ceratopsians. The paratype has nine articulated 

 centra, of which the anterior six are regarded as true sacrals and the posterior two as true caudals. 

 The remaining intermediate vertebra between tail and sacrum constitutes a modified caudal that 

 functions to a certain extent as a sacral and can therefore be designated a sacrocaudal. 



"The sacrum of the paratype, including the sacrodorsal and the sacrocaudal, had eight centra 

 with sutured articular ends whereas Monoclonius and Triceratops had ten such vertebrae. 48 [This 

 is, however, a juvenile condition and not necessarily a generic distinction, for it is probable that at 

 least two more vertebrae would later be added to the sacral series, bringing the number up to the 

 standard ten.] 



". . . The first sacral may be recognized at once by the great transverse breadth of the centrum, 

 its flattened ventral surface, and the inferior position of the posterior articular facets for the second 

 sacral rib ... . The second sacral centrum is also distinguished from all others by the much greater 

 breadth of the anterior as compared with the posterior extremity. The centra of the remaining 

 sacrals are of about equal dimensions, higher than wide, constricted medially, with slightly flattened 

 inferior surfaces. 



"A second specimen (No. 8072, U.S.N. M.), however, shows centra that are broader than high 

 and that have a shallow longitudinal depression on their inferior surfaces. The differences are in 

 all probability due to crushing. The first three vertebrae of the sacral region . . . are decidedly 

 heavier than any that succeed them. The sacral ribs are borne jointly by all the centra. . . . The 

 second and strongest sacral rib of the series articulates jointly with the first and second dorsals 

 [sacrals] low down on the sides of the centra, the inferior surface of the rib being on a level with 

 the ventral surface of the sacrals. . . . The succeeding ribs articulate with facets that are but little 

 below the level of the floor of the neural canal. 



". . . The few spinous processes are exceedingly short thin plates of bone with little transverse 

 thickening of their superior extremities. That these were united into a bony plate is indicated by a 

 detached sacral process belonging to another individual (No. 8072, U.S.N. M.), which shows sutural 

 edges fore and aft that continue to the top of the spine. The same condition prevails on the spine 

 of the fifth sacral, ... so it would appear that this plate may have been continuous from the first to 

 the fifth, much as it is in Triceratops. [The sacrum of Triceratops figured by Marsh, and now part of 

 the mounted animal in the United States National Museum, is peculiar in that the first spinous 

 process is free, the second to fifth fused into a plate, and the remaining ones free; that of sacral VI 

 slants aft so as to make a wide-angled notch between it and number five. Compare Monoclonius 

 (Centrosaurus), Fig. 17.] The relative shortness of the spines in Brachyceratops would at once dis- 

 tinguish its sacrum from that of the former genus. [It is conceivable, however, that as a mechanical 

 adjustment to increasing weight the spinous processes would lengthen with age.] 



"As in Triceratops the spinal cord appears to have been only slightly enlarged in the anterior 

 sacral region. The diapophyses are comparatively weak and are given off on a horizontal plane from 

 the neural arches, with thin ends directed forward. . . . Their inferior borders extend obliquely down- 

 ward and inward and present a narrow sutural edge for articulation with the upper and inner borders 

 of the sacral ribs. . . . Nearly all of the transverse processes are missing, but their point of origin, as 

 shown by the broken surfaces, is indicated . . . ." 



A complete series of caudal vertebrae, 50 in number, was discovered for the first time in 

 Brachyceratops. Since then a complete series of 45 has been found in Protoceratops and of 38 in 

 Anchiceratops. Thus the number is greater than in any other ceratopsian genus, as evidenced either 

 by actual count or by estimate. "The greater number were found articulated, and such displace- 

 ment as existed was so slight that there can be no question that they represent a consecutive 

 series. . . . 



"Gilmore, C. W., 1917, pp. 21-23. 



