24 THE CEKATOPSIA. 



the lachrymal and postero-superiorly with the postfrontal. The jugal thus forms the external 

 surface of the skull throughout the anterior portion of that area lying between the orbit and 

 the infratemporal or lateral temporal fossa. 



The relations of the various elements constituting the quadratojugal arch to one another 

 and to the surrounding bones are well shown in figs. 6, 10, 24, and 26, and in PI. XLIV. 



THE FRONTAL REGION. 



The prefrontals, frontals, and postfrontals are all present, though the relative importance 

 of each is somewhat different from that which'is usual in the reptilian skull. The characters 

 of these elements are best seen in the type of Triceratops serratus, No. 1823 of the Yale Museum 

 collection, and I shall base my description of these elements on that skull. 



The frontals are small and become coossified in early life medially with one another, later- 

 ally with the prefrontals, and posteriorly with the postfrontals, so that in the skulls of old indi- 

 viduals the sutures between all these bones are completely closed, the whole forming then a 

 single bone roofing over the entire top of the skull between the parietals and nasals, constituting 

 about two-thirds of the orbital borders and culminating in the transversely placed and powerful 

 pair of supraorbital horn cores. In the types of Triceratops flabellatus and T. serratus, Nos. 

 1821 and 1823 of the Yale Museum collections, most of the sutures between the different ele- 

 ments of the frontal region are still open, so that in these skulls it is still possible to determine 

 the form of the different frontal bones. The true frontals are abruptly truncated posteriorly 

 in the type of T. serratus, No. 1823, but anteriorly they are deeply emarginate and receive the 

 posterior median projection of the nasals. The frontals form no portion of the orbital border, 

 nor do they contribute to the formation of the supraorbital horn cores. 



The prefrontals are placed alongside of and external to the frontals, which they about equal 

 in size. Supero-posteriorly they articulate with the postfrontals and immediately beneath the 

 postfrontal suture they form the extremely thick anterior border of the orbit, which in this 

 region is projected far beyond the general surface of the skull. Inferiorly they articulate with 

 the lachrymals, while anteriorly they are in contact with the nasals. 



The postfrontals in the Ceratopsia have become of unusual importance. They are several 

 times larger than the frontals and prefrontals combined. They form more than one-half the 

 orbital border and are extremely massive posteriorly. They alone give origin to the so-called 

 frontal horn cores, which might therefore be more appropriately called the postfrontal or supra- 

 orbital horn cores. The latter name is preferred by the present author. Anteriorly they are 

 in contact with the frontals and prefrontals, posteriorly with the parietals and squamosals, 

 while immediately beneath the center of the orbits they oppose the jugals. From beneath the 

 postfrontals receive support from both the supraoccipital and the alisphenoid. On the median 

 line and just in advance of the parietal suture a large foramen is usually found piercing the 

 external wall of the postfrontals. It has been called by Professor Marsh the pineal foramen. a 

 This foramen appears to have communicated with the large cavities in the postfrontals at the 

 base of the horn cores and with certain other smaller cavities in these bones and between them 

 and the supraoccipitals. Its functions are not known, but it is not unlikely that it served for the 

 transmission of nerves and nutrient blood vessels. In some skulls of very old individuals it is 

 almost or entirely closed; the latter is the case in the type of Triceratops prorsus, No. 1822, 

 Yale Museum. 



The base of the supraorbital or postfrontal horn core is invaded by a very large cavity with 

 comparatively thin walls of bone. Between the horn cores there are a number of smaller cavi- 

 ties situated within the body of the postfrontals, as shown in fig. 24. A number of somewhat 

 larger cavities lie between the supraoccipital and the posterior portion of the postfrontals and 

 anterior portion of the parietals and squamosals. There appears to be considerable individual 

 variation in the number and relative size, form, and position of these various cavities. 



a This is in no sense a "pineal foramen," as it leads into the postfrontal sinuses and noHnto the brain case. It is a temporary opening, gen- 

 erally closing in older individuals, and hence is analogous to the fontanelle in the skull of the human infant. A more accurate term would be 

 postfrontal fontanelle , and as it was evidently not permanent the idea of its transmitting nerves and blood vessels seems hardly tenable. 

 (See fig. 33.)— R. S. L. 



