164 THE CERATOPSIA. 



5. Triceratops (including Sterrholophus) . — In Triceratops this phylum reaches its culmina- 

 tion in specialization and size, including some most formidable species. 



The supraorbital horns reach their maximum development in Triceratops, while the 

 nasal horn is in process of reduction and the broadly expanded frill, though not so large 

 actually or relatively as that of Torosaurus, was much more efficient for protection because 

 of its vastly greater strength. 



The rostral bone is better developed than in Diceratops, being especially heavier in older 

 individuals, and there is generally a sharp cutting edge on its inferior face. The nasal horn, 

 while tending to reduce, is of moderate length in the more generalized species and is directed 

 forward, so that its posterior face rises but little above the level of the upper surface of the 

 nasal bones, in sharp contrast to that of Monoclonius. The nasal horn core is generally very 

 rugose. The supraorbital horn cores are slender to robust, ovate in section as contrasted 

 with the triangular cross section of the rudimentary horn cores in Monoclonius and the nearly 

 circular section of those of Diceratops. In Triceratops the horn cores point upward and forward 

 at an angle of 45°, whereas in Diceratops they are nearly erect. They range in length from 

 short in T. brevicornus to very long in T. calicornis. The orbit is generally elliptical in shape 

 with the long axis somewhat diagonal, running from above downward and forward, therein 

 agreeing with Diceratops rather than with Monoclonius. 



The parietals are convex laterally and somewhat concave along the long axis, much 

 expanded posteriorly, but narrowing toward the forward end. They are thick along the more 

 or less elevated and rugose middle line and around their posterior border, but very thin in 

 the center of each side, though there are no traces of fontanelles. It is mainly in this last 

 feature that the parietals differ from those of the other genera. The squamosals are stout and 

 broad, constituting at least half of the area of the frill. The superior surface of the frill, and, 

 in some aged individuals, the margin of the lower side as well, bears vascular impressions, 

 showing it to have been sheathed in a horny covering. These impressions are much more 

 pronounced than in either Monoclonius or Centrosaurus. 



Hatcher has made no final disposition of the genus Sterrliolophus either in his completed 

 manuscript or in his notes, but one is justified in the assumption that he believed the genus 

 to be synonymous with Triceratops on the ground that all of the distinctive features which 

 would serve to separate the genera he considered as juvenile characteristics. 



The type consists of disarticulated skull and other skeletal portions of an immature 

 individual and differs from a typical specimen in the entire absence of vascular impressions 

 from the frill. Those on the under surface are seen only in aged individuals, as in the type of 

 Triceratops prorsus (PI. XXXIII), and it may well be that their appearance on the superior 

 surface of the skull is a strictly adult feature. 



Another distinction between the skulls of Triceratops Jiorridus and Sterrholophus flabellatus 

 is shown in figs. 9 and 27, pages 18 and 30. In the former the olfactory foramina are paired, 

 whereas in the latter there is bu;t a single foramen. But, as Hatcher says, "it is probable that 

 the median septum was present as cartilage and was lost in maceration. It is very unlikely that 

 there was in any of these dinosaurs a single olfactory nerve." Another distinction between 

 Sterrholophus flabellatus and some species of Triceratops is in the position of the lachrymal 

 foramen, which is entirely within the maxillary, not as in other species between the maxillary 

 and nasal. In this feature Triceratops prorsus agrees with Sterrliolophus flabellatus, while in 

 T. serratus the other condition obtains, T. brevicornus having the foramen between the nasals 

 and maxillary but embraced by an ascending process of the maxillary in its anterior moiety, 

 a condition transitional between that of the other two. This character can thus be considered 

 of specific value only. It would therefore seem wise, in the light of our present knowledge, 

 to consider the newer generic name Sterrholophus as a synonym of Triceratops. 



The main skeletal comparisons between Triceratops, Agathaumas, and Monoclonius which 

 it is possible to make lie in the sacrum and ilia, in which Triceratops shows a further advance- 

 ment over Agathaumas. The general structure of the sacrum agrees with that of the latter 

 in having ten vertebrae. Of these four bear sacral ribs, which coalesce to form the longitudinal 



