MONOCLONIUS CEASSUS. 



75 



lation with the parietal. Externally this assumes the nature of a wide, deep pocket formed by 

 the posterior extension of the superior and inferior walls of the bone. Into this pocket the 

 lateral angle of the median bar of the parietal fitted. At about the middle of the posterior 

 margin of the postfrontal for a very short distance it presents a free, thin border, indicating 

 the presence of a small foramen opening into a large cavity beneath the surface of the post- 

 frontal, which is here comparatively thin. The posterior margin of the deflected lateral 

 portion of this bone presents in part a fractured surface and in part a suture for articulation 

 with the squamosal. Inferiorly there is a short sutural surface for articulation with the jugal, 

 though for the most part this is not shown. The usual articulation between this bone and the 

 parietal, squamosal, and jugal seems to have been by overlap and underlap rather than by 

 direct interlocking sutural contact. The antero-external border of the frontal is thin and 

 presents a free edge, which appears to have overlapped the prefrontal. 



The sacrum. — This (No. 3998a, American Museum of Natural History) is composed of 10 

 vertebrae, if we include all those coossified with the true sacrals. If, however, we determine 

 the number of sacrals by the number of sacral ribs there are but 8 sacrals, and of these only 4 



Pig. 77.— A, Oblique inferior view of sacrum of type of Monoclonius cmssus Cope (No. 399Sa, American Museum of Natural History); 

 B, oblique superior view of same, a, Anterior end; p. posterior; ab, acetabular bar; /, foramina between sacral ribs, centra, and acetab- 

 ular bar. One-eighth natural size. 



or perhaps 5 had the sacral ribs united distally into a longitudinal bar, both giving support 

 to the ilium and entering into the construction of the acetabulum. The general form of the 

 sacrum is much obscured by distortion, due to crushing. The posterior sacral ribs are for the 

 most part incomplete, and none of the neural spines are entire. Many of the diapophyses are 

 also injured, but otherwise the sacrum is complete and in a good state of preservation for a 

 dinosaurian sacrum. The centra are all of medium length and somewhat constricted medially. 

 Commencing with the fourth and terminating with the ninth, the inferior surface is broad and 

 shows a median shallow groove somewhat more marked at the junctions of the several centra. 

 Save the first and last all the centra have the vertical diameter exceeding the transverse, 

 though these proportions have undoubtedly been materially affected by crushing. The first 

 three centra are decidedly heavier than any of the succeeding ones, and the last is much com- 

 pressed and proportionately stouter than those immediately preceding it. The first two sacrals 

 do not bear ribs, although the first sacral rib springs jointly from the second and the third 



