42 



THE CERATOPSIA 



Cervical IX 



(Fig. 9) 



Cervical IX corresponds more nearly to Hatcher's figure. In this bone the centrum is still 

 biconcave, with the sides more deeply constricted than in M. crassus. The centrum is more evenly 

 rounded, lacking the flattened lower surface seen in cervical VIII. The neural canal is still large 



az 



Y.PM. 20J5 



Yic. 9. Cervical IX of Monoclonius (Centrosaurus) fiexus. Left lateral, and anterior views, Y\ natural size, az, anterior 



zygapophysis; d, diapophysis; n, neural canal; fz, posterior zygapophysis; s, neural spine; t, tubercular facet. 



and somewhat triangular in section except for a rounded apex. The capitular facet is hardly visible 

 but is still borne on the centrum, while the distinct tubercular facet lies at the end of the diapophysis. 



Dorsal Vertebrae 



One marked peculiarity of the dorsal vertebrae (Fig. 10) of this specimen, which may be a 

 specific character, is the more backward slant of the spinous processes as compared either with 

 M. crassus or C. naskomus (Figs. 11, 12) in which they are more nearly erect. This_ involves the 

 entire neural arch and becomes more marked in the posterior vertebrae, especially in dorsal VI 

 (see Fig. 13). In the Yale specimen the lateral constrictions are also deeper. The diapophyses are 

 triangular in section, with the base of the triangle uppermost and the other sides concave. 



K P.M. 2015 



Fig. 10. — Dorsal II of Monoclonius {Centrosaurus) flexus. Left lateral, and anterior views, l /i natural size, az, anterior 

 zygapophysis; d, diapophysis; h, capitular facet; n, neural canal; f, pleurocoele; fz, posterior zygapophysis; /, neural 

 spine; /, tubercular facet. 



