162 THE CEEATOPSIA. 



Of the two most primitive genera Monoclonius seems to be the more generalized and repre- 

 sents the earliest known stage in the evolution of the Ceratopsia. Because of the gap 

 between the Judith River beds and those of the Laramie the series is by no means complete, 

 nor are we yet aware of the characteristics of pre- Judith River ancestors. The earliest known 

 Ceratopsia are endowed with the main distinguishing characters, the horns and parietal crest. 



With the exception of the genus Agathaumas, of which the skeleton only is known, the 

 more important taxonomic characters are found in the cranium, and the genera can readily 

 be differentiated without recourse to the skeletal features; in fact, in four out of the seven 

 genera these are unknown. 



REVISION OF GENERA. 



Taking the six genera recognized by Hatcher and one erected later by Lambe, which Hatcher 

 would undoubtedly have accepted as valid, the main generic comparisons and contrasts are as 

 follows: 



I. THE MONOCLONIUS-TRICERATOPS PHYLUM. 



1. Monoclonius, the most primitive genus, is comparatively small, and has three horn 

 cores, the nasal being long, powerful, and curved backward. The supraorbitals are diminutive, 

 flattened on the outer surface but very convex on that toward the median line, so that the basal 

 section is almost triangular. The horns are sharp pointed. The orbit is nearly circular and 

 in at least one species has a heavy rugose border. The frill is made up largely of the coalesced 

 parietals, the squamosals being somewhat triangular, plate-like bones, short and proportion- 

 ately broad. The margin of the crest is crenulated, but the prominences do not seem to have 

 arisen from separate ossifications, as in the succeeding genera of this phylum. The parietals are 

 widely fenestrated — in fact, they do not entirely surround the fenestra?, and the median bar is 

 broad and thin in contrast to the much narrower bar in Ceratops. Of the skeletal characters 

 those of greatest taxonomic value are found in the sacrum. The number of coalesced vertebrae 

 in the entire sacrum is ten; of these eight bore sacral ribs, of which four pairs united distally on 

 either side into a longitudinal bar for the support of the ilium. The centra were of medium 

 length, constricted medially, and the sacral ribs arose directly opposite the points of articu- 

 lation, so that each rib bore equally on two contiguous centra. The ilium is rather long and 

 slender, and the deflected margin posterior to the ischiac peduncle is produced into a somewhat 

 angular prominence. The blade is rounded anteriorly, while posteriorly it is elongate and 

 narrows presumably to a point, though the extremity is not preserved. The blade, when the 

 bone is in position, is horizontal, as in other Ceratopsia. In contrast with the ilium of Aga- 

 thaumas that of Monoclonius is smaller and proportionately more slender, and the deflected 

 external margin is more pronounced. 



2. Centrosaurus. — In Centrosaurus the nasal horn is straight, laterally compressed so as 

 to be lenticular in section, and somewhat similar to that of Monoclonius sphenocerus. The 

 coalesced parietals entirely surround the large, oval fontanelles and the median bar is very 

 heavy, especially between the peculiar inward-curved processes at its posterior border, but 

 thins perceptibly toward the anterior portion of the frill. On either side the fontanelle is 

 inclosed in a thin extension of bone. In addition to the two curved posterior prominences there 

 are seven others, separated by emarginations, the posterior ones bearing separately ossified 

 epoccipital bones as in Triceratops. The squamosals are short, as in Monoclonius, being 

 confined to the antero-external angles of the frill. 



3. Agathaumas. — The genus Agathaumas, so far as known from skeletal fragments, is 

 intermediate in character between Monoclonius and Triceratops. It comes from the lower 

 Laramie, near Black Buttes Station, Wyoming. In Agathaumas the main distinctive charac- 

 ters are of necessity derived from the sacrum, the ilia, the dorsal vertebrae, and the ribs. The 

 sacrum has five true sacral vertebras, probably with one presacral and four caudals, making 

 ten in all, as in Monoclonius. In the type specimen, which was not fully mature, there are 

 four pairs of sacral ribs, arising more directly from the articulations than in Triceratops, yet r 

 with the exception of the first, not so much so as in Monoclonius, being in a condition 



