GENERIC AND SPECIFIC SUMMARY. 165 



bar for articulation with the ilium. The first pair of ribs agrees with those of both above-men- 

 tioned genera in springing from the point of articulation between vertebras 2 and 3, bearing 

 equally upon each, while the succeeding sacral ribs arising behind the articulation bear very 

 little upon the preceding centrum at all. A reference to figs. 53 and 77 (pp. 52 and 75) and 

 Pis. X and XXV (pp. 218 and 248) will make clear these comparisons and contrasts. 



The ilium in Triceratops is broad and elongated, much expanded anteriorly and tapering 

 to a somewhat thickened point behind, whereas in Agaihaumas the posterior extremity is thin. 

 The external border is thin and smooth in contrast to the variably thickened margin in the 

 latter genus. The outline as viewed from above is less regular than in Agathaumas, which is 

 due to a greater prominence of the deflected border above the ischiac peduncle. In Triceratops 

 the internal margin is greatly thickened, forming the superior border of the acetabulum and 

 strengthening the peduncles. Compare figs. 55, 61, and 82. 



II. THE CERATOPS-TOROSAITRUS PHYLUM. 



Of the second phylum but two genera are as yet known, Ceratops and Torosaurus, agreeing 

 in the possession of large, persistently open parietal fenestra? and in the general conformation 

 of the squamosal, which, while not so saber-like in Ceratops as in Torosaurus, exhibits a strong 

 tendency to become so, in sharp distinction with the short, broad, triangular squamosals of 

 Monoclonius. 



The members of this phylum also agree in the absence of separately ossified epoccipitals, 

 the crenulated effect of the margin of the frill being produced by prominences which do not 

 arise as separate ossifications. An exception to this is found in the type specimen of Ceratops 

 (Monoclonius) recurvicornis Cope, referred to Ceratops by Hatcher but possibly representing 

 a new genus, a member of the Triceratops phylum. 



1. Ceratops, a Judith River genus, is known only from portions of the skull, no other 

 skeletal elements having been discovered. Our knowledge of the nasal horn core is imperfect, 

 as it is derived only from the specimen of Ceratops recurvicornis mentioned above. In this 

 specimen the horn core is massive, compressed distally, and curves forward instead of back- 

 ward, as in the contemporary genus Monoclonius (see fig. 3). Another isolated horn core, 

 figured in Lambe's memoir (PI. XVII, fig. 1 = P1. XVIII, fig. 1 of this monograph), was 

 referred by that author with a query to Monoclonius dawsoni Lambe. Hatcher claims that 

 it curves forward instead of backward, which would seem to place it in the genus under con- 

 sideration, though under what species can not be determined. 



The supraorbital horn cores show a greater degree of development than in Monoclonius, 

 but are apparently still smaller and less massive than the nasal core. They are more nearly 

 circular in cross section, being subtriangular near the base, and are curved forward and upward 

 and, in one species, C. wontanus, strongly outward (fig. 103). 



The orbit is oval, with its long diameter nearly vertical as contrasted with the circular 

 orbit of Monoclonius and the more nearly horizontal oval of Torosaurus. 



The coalesced parietals are reduced to a slender, median bar, subcylindrical in mid-length, 

 and narrow postero-lateral processes, which only partially surround the large elliptical 

 fontanelles. 



The squamosals, which overlap the outer portions of the parietals, are triangular and 

 more elongate than in Monoclonius, tending toward the form of squamosal found in Torosaurus. 



2. Torosaurus. — The proportions of the Torosaurus cranium at once separate it from that 

 of any other genus, especially from its immediate contemporaries, for while in Triceratops the 

 frill and facial region are well balanced, in Torosaurus the huge crest entirely overshadows 

 the abbreviated anterior portion of the skull. 



The nasal horn core in the type species is broad at the base and the sharp apex is directed 

 upward and forward. The supraorbital horn cores are larger, as in all Laramie Ceratopsia, 

 and ovate in basal section, with the broader end of the oval to the rear. The cores are directed 

 upward, outward, and forward. Thus the nasal horn core is relatively smaller while the supra- 



