32 THE CERATOPSIA. 



THE SUPRAORBITAL HORN CORES. 



Next to the frill the supraorbital horn cores form the most striking feature in the skull 

 of the Ceratopsia. They rise from the superior and lateral surfaces of the postfrontals and 

 may be considered as outgrowths from those bones, the frontals and prefrontals entering but 

 little, if at all, into their composition. They are placed transversely one on either side above 

 the orbits and are directed upward, outward, and forward with a slight curve. They are 

 acutely pointed and vary much in length and form, even in the same genus, and still more 

 so in the different genera. In Monoclonius and Ceratops they are frequently not more than 

 15 or 20 centimeters in length, while in some of the species of Triceratops they occasionally 

 attain a length of almost or quite a meter. They vary also in form in the different genera 

 and in size as compared with that of the nasal horn core. In Ceratops and Monoclonius they 

 appear as a rule to have been smaller than the nasal horn core, but in the later and larger 

 forms included in the genera Triceratops, Torosaurus, etc., they greatly exceed the nasal horn 

 cores in size. The supraorbital horn cores of Triceratops are ovate in cross section, the antero- 

 posterior diameter being the longer, the posterior surface broadly rounded and the anterior 

 narrowed so as to form the apex of the ovate figure formed by the cross section. They become 

 more nearly circular toward the apex. The external surface is somewhat flattened, especially 

 near the base. To support the horn cores the postfrontals are extremely thickened behind 

 the orbits, and are supported beneath by the jugals, squamosals, parietals, and supraoccipitals. 

 As with the horn cores in the cavicorn Mammalia, the supraorbital horn cores in the Ceratopsia 

 are hollow at the base, and in some of the larger forms these cavities are very large, being 

 at the base of the horn as much as 15 centimeters in diameter and extending upward, though 

 rapidly diminishing in diameter, for a distance of from 25 to 30 centimeters. These cavities 

 are confluent with other and smaller cavities found in the massive postfrontals at the base 

 of the horn cores. Above the large cavity at the base the central mass of the horn core is very 

 cancellous in structure, while toward the peripherj^ the bone becomes firmer, though con- 

 tinuing somewhat porous throughout its entire thickness. The presence of this cavity at 

 the base of the supraorbital horn core in the Ceratopsia is one of the many examples in the 

 animal kingdom of the application of that mechanical principle which gives the greatest pos- 

 sible increase of strength and superficial area with the least possible weight, and which is 

 most admirably illustrated in the cervical and dorsal vertebra? of the sauropod dinosaurs. 

 The external surface of the supraorbital horn cores is everywhere rugose and marked with 

 deep vascular impressions, probably for the lodgment and protection of the blood vessels 

 and nerves which were inclosed between the bony mass and the sheath of horn with which 

 the former was in life undoubtedly incased. In the type of T. {Sterrholophus) flabellatus (No. 

 1821, Yale Museum), when discovered, a portion of the investing horny material was still in 

 place about the left horn core, though in such a decomposed condition that it was impossible 

 to preserve it. 



THE NASAL HORN CORE. 



There is the greatest diversity in the form and size of the nasal horn core in the various 

 genera and species of this group. In some of the earlier, smaller, and more primitive forms 

 of the Judith River beds of Canada and Montana these organs are not only relatively but 

 absolutely longer than in the later, larger, and more specialized forms from the Laramie deposits 

 of central eastern Wyoming. Moreover, while in these later forms the supraorbital horn 

 cores are always much longer and more robust than the nasal horn cores, in the earlier 

 forms, such as Monoclonius recurvicornis , and very probably M. sphenocerus, the nasal horn 

 cores are absolutely larger than the supraorbital. Among the larger and later forms from 

 the Laramie the nasal horn core seems to attain its greatest length in Triceratops prorsus, 

 where, in the type (No. 1822, Yale Museum), it is acute, nearly circular in cross section, and 

 has a length of about 20 centimeters. No nasal horn cores have yet been found associated 

 with any of these later Laramie forms equaling in length that of the type of Monoclonius 

 sphenocerus Cope from the Judith River beds of Montana. While these horn cores are supported 



