4 g THE CERATOPSIA 



buttresses of bone which, together with the diapophyses, form a T- or L-shaped cross section. All 

 of them meet the ilium except those of sacral X. 



The sacrum of C. cutleri type differs from that of the Yale specimen mainly in the forward 

 extension of the ilia in front of the sacrum. In both, the rib of the dorso-sacral is a slender bone 

 fused with the under side of the blade of the ilium near its anterior end; these ribs in C. cutlen also 

 curve forward much more than they do in the Yale specimen, which seems to show that the ilia in 

 both specimens are essentially complete. In the Yale specimen the diapophyses of sacral X do not 

 meet the ilia, in C. cutleri they do, as seen from below. The visible differences between the pelves 

 of the Yale Centrosaurus and C. cutleri are slight. To what extent they are due to preservation is 

 not clear. They are otherwise perhaps accountable as individual, perhaps sexual variations, and are 

 hardly of specific value. 



Measurements of the Pelvis 



C. flexus Y.P.M. C. nasicornus 



Length of entire sacrum over zygapophyses, including dorso-sacral 840 mm. 800 mm. 



Width over anterior diapophyses 252 



Width over diapophyses of IV 237 



Width over diapophyses of VII 276 



Width over diapophyses of X 226 



Width across ilia, anterior end 75i 



Width across ilia, over ischiac peduncle 630 



Width across ilia, posterior end ~~ 



Length of right ilium, as preserved 960 1060 



Length of left ilium, as preserved 968 



Length often sacral centra ™3 



Width across acetabular bar at pubic peduncle 322 



Length of ilium in C. cutleri (estimated from photograph ) 1 1 24 



It is only from below (Fig. 18) that the dorso-sacral is clearly differentiated from the sacrum 

 proper, for here the centrum is free from the following ones, being separated from sacral I by at 

 least 1 6 mm. of matrix at its lower margin. There are normally ten sacral centra, although the line 

 of demarcation between the ninth and tenth is not clearly indicated in the present condition of the 

 Yale specimen. In the C. cutleri type all are distinct. 



The centrum of the dorso-sacral is constricted medially but possesses no pleurocoeles. Ventrally 

 it is flattened and but slightly concave fore and aft. The parapophyses are distinctly indicated and are 

 fused with the horizontally flattened diapophyses which, at their distal ends, are coossified with the 

 ilia. In addition, the dorso-sacral bears a pair of slender, slightly curved ribs which proximally abut 

 against the slightly dilated end of the parapophyses, with which they are in line. Distally these ribs 

 curve slightly forward and unite with the ilia below and a little behind the anterior margin of the 

 latter as preserved. On the left side, the rib extends to the outer margin of the ilium, but the tip is 

 not present and may have jutted slightly beyond the limit of the ilium. The centra of sacrals I-III 

 are broad, and flattened inferiorly, being somewhat trihedral in cross section. Numbers V-VII are 

 distinctly narrower and are constricted medially. Sacrals VIII-X widen again, the median constric- 

 tion disappearing in the last. Beginning about the middle of centrum III is a deep median groove 

 which is deepest in the anterior part of IV. It soon fades out, however, but can still be seen at the 

 union of VI and VII. In Triceratofs this groove begins with centrum II and extends the entire 

 length of the sacrum, being widest in II-IV, where it is also most marked in the Yale Centrosaurus. 

 In M. crassus, commencing with the fourth and terminating with the ninth, the inferior surface of 

 the sacrum is broad and shows a median shallow groove somewhat more marked at the junctions 

 of the several centra. The Yale specimen, therefore, differs from both Triceratofs and M. crassus^ 

 in the fore and aft limits of the ventral median groove, which is noticeably shorter. The C. cutleri 

 type is more like M. crassus in this respect. 



