GENERIC AND SPECIFIC SUMMARY. 163 



transitional between that in each. The four sacral ribs coalesce distally to form the horizontal 

 bar for articulation with the ilia, but in the present instance the bar of each side is not 

 continuous, the ribs being, as Cope expresses it, united in pairs. It is possible that continuity 

 would have come later in life had the creature reached maturity. The centra of the true 

 saerals decrease in size regularly from first to last, as in Monoclonius and Triceratops, the 

 first and second being particularly broad and short. They are somewhat more elongate than 

 the dorsal centra, the vertical and transverse axes being more nearly equal. Perhaps the 

 most important distinction is the point of origin of the sacral ribs. There was evidently a 

 pleuro-diapophysial connection with the ilium, as in Monoclonius and Triceratops, but the 

 diapophyses are not preserved. The dorsal vertebra? exhibit no marked distinction from 

 those of the other genera. 



In Agathaumas the blade of the ilium is much extended fore and aft, the anterior end 

 being truncated and the hinder extremity narrow and elongate. The external margin is 

 moderately thick throughout its length and much thicker near the extremities and in the 

 deflected region above the ischiac peduncle. The internal margin is thin except in the region 

 of articulation with the sacral diapophyses. A broad, convex ridge of bone extends diagonally 

 across the anterior blade of the ilium to its antero-external angle, inclosing in front a deep 

 depression just external to the acetabulum. 



The ilium is actually larger and relatively shorter and broader than that of Monoclonius, 

 and the deflection of its external margin is less pronounced. It differs from the ilium of 

 Triceratops in being more slender and in having occasional thickenings of the external border, 

 whereas in Triceratops the margin is more uniformly thin. The posterior end of the Aga- 

 thaumas ilium is thin, while that of Triceratops is thickened. 



The ilium of Agathaumas is transitional between those of the contrasted genera in general 

 proportions, especially the relative length and breadth. 



4. Diceratops. — This genus is known from a complete skull and, although unquestionably 

 related to the Monoclonius- Triceratops phylum, its serial order is somewhat difficult to conjecture. 

 By the absence of a nasal horn core it resembles Triceratops obtusus, though evidently not 

 synonymous therewith. The fenestrated parietals would seem to point to primitive conditions 

 until one notes the presence of similar fenestra?" in the squamosals, a character which here 

 appears for the first time. This, together with the fact that the squamosal fenestra are of 

 unequal size — which may also have been true of those of the parietals, as only the right is 

 preserved — leads one to conjecture whether they may not have been secondarily acquired 

 and together with the vestigial nasal horn, may not be evidences of high specialization from 

 some Triceratops-like ancestor. The main diagnostic characters wherein Diceratops differs 

 from Monoclonius are the reduced nasal horn, the vastly increased supraorbital horns, the 

 elliptical orbit, the well-developed parietals with their small fontanelles(?), the more elongate 

 fenestrated (?) squamosals, and the separately ossified epoccipital bones. 



From Triceratops, its nearest ally, it may be distinguished mainly by the much smaller 

 rostral bone; by the absence of the nasal horn, which in all of the species of Triceratops 

 except T. obtusus is fairly well developed; by the very erect, short, robust supraorbital 

 horn cores, which seem to take their origin much farther back with relation to the orbit; by 

 the concavity of the frontal region between the orbits; and, finally, by the peculiar form of 

 the persistent postfrontal (pineal) fontanelle suggestive of that of the genus Torosaurus. The 

 parietals are Triceratops-like except for the small fenestra? on either side of the median line, 

 while the squamosals, aside from the unique fenestra?., differ from those of Triceratops in 

 the conformation of the inferior border, which lacks the quadrate notch. Another distinctive 

 feature is in the very erect position of the descending process of the jugal, which is directed 

 slightly forward instead of downward and backward as in Triceratops. 



a The writer is now firmly convinced that all of these apertures through the frill of Diceratops are pathologic, having been caused 

 either by wounds or by disease (Am. Jour. Sci., 4th ser., vol. 20, pp. 419-422). Similar perforations occur in the right frontal of the type of 

 Triceratops serratus (p. 124) and in the squamosal of that of T. elatus (p. 136). Mr. C. W. Gilmore, who prepared the type, is not sure of 

 the parietal perforation, but as no bone adhered to the matrix at that point the opening was allowed to remain. 



