GENERAL MORPHOLOGY 



Mr. Hatcher's 1 study of the osteology of Triceratops leaves little to be desired. There have 

 been, however, a few new elements discovered since his time, such as the hyoid, sternal, and sclerotic 

 plates, which will be discussed in their proper place. In addition, studies have been made of the 

 cranial musculature and a good deal of argument has arisen concerning the homologies of certain 

 elements in the skull. Hatcher also discussed in such detail as was then possible the skeleton of 

 Cope's type of Monoclonius crassus. Because of the abundance and perfection of material which 

 has come to light in the Red Deer region of Alberta, including at least seven nearly complete skele- 

 tons representing four genera and two geological horizons, it is now deemed wise to describe anew 

 the morphology of the genus Monoclonius {Centrosaurus), basing the description on the skeleton at 

 Yale, recently purchased from the American Museum of Natural History (No. 2015 Y.P.M.). 



MONOCLONIUS (CENTROSAURUS) FLEXUS 2 Brown 



The animal was much smaller than Triceratops and the limb bones, especially, lack the robust- 

 ness of that genus, although aside from the skull the skeletal elements are otherwise quite similar. 



AXIAL SKELETON 

 SKULL 



The skull (Fig. 4) of the Yale specimen resembles that of the type of Centrosaurus flexus 

 Brown, differing only in minor details such as having two forwardly directed processes over the 

 parietal fenestrae instead of but one, and in the nearer approach of the inwardly curved hook-like 

 processes at the rear of the crest. These are shown to be matters of individual variation and appar- 

 ently have little or no specific value. 



In Brown's 3 description of C. flexus the following points are noted for comparison: He speaks 

 of the nasal horn being longer than in any other described species of the genus. This was true at 

 that time, but his subsequently described C. nasicornus has a longer one although straight rather than 

 curved forward. The nasal horn in the C. flexus type is described as ovate in section at the base with 

 the broad end of the oval toward the rear, with which the Yale specimen agrees. 



The horn arises from the nasals at their mid-length in both specimens so that its center lies 

 directly above the posterior border of the nares. This is a character common to all early genera, in 

 contrast with those of the Lance in which the horn lies over the anterior rim of the nares. In 

 C. flexus the horn curves forward; in Monoclonius dawsoni Lambe it curves backward with about 

 the same degree of curvature, while in M. sphenocerus Cope and C. nasicornus Brown it is straight 

 and erect. 



The supraorbital horns are rudimentary as compared with those of Triceratops. They are con- 

 siderably larger than in Monoclonius dawsoni, and in both the type of C. flexus and the Yale speci- 

 men they are asymmetrical, that of the left side being in each skull considerably higher than the right. 

 In the Yale specimen the left horn core is triangular, viewed from without, with a flattened outer 

 face, and less rugose than in the type. On the right side there is only a rough, rounded area where 

 the horn should be. Brown thinks it possible that the horns may have been derived from separate 

 centers of ossification and that the right one has been lost, leaving only the rugose elevated portion 

 of the postfrontal forming its base of attachment. In the type of Styracosaurus albertensis (PI. 

 VIII, B) in the Ottawa museum there are distinct sutural surfaces showing where the horn cores 



1 Hatcher, Marsh, Lull, 1907, Chapter III. 



2 For the sake of simplicity, during this chapter on morphology the term Centrosaurus will be used for all Belly River 

 species under the genus Monoclonius. 



3 Brown, B., 1914, B. 



