. The Ceratopsia. Ceratopsia. THE SKULL. 33 by the nasals, and in old animals become firmly coossified with them, they nevertheless have their origin in separate and distinct centers of ossification. Moreover, since in young indi- viduals ever}7- nasal horn core is seen to have had its origin in a single median center of ossification rather than in two distinct lateral centers placed one beside the other, it is evident that this horn core is in reality morphologically quite distinct from the nasals. In this respect the nasal horn cores differ greatly horn the supraorbital horn cores, which are
. The Ceratopsia. Ceratopsia. THE SKULL. 33 by the nasals, and in old animals become firmly coossified with them, they nevertheless have their origin in separate and distinct centers of ossification. Moreover, since in young indi- viduals ever}7- nasal horn core is seen to have had its origin in a single median center of ossification rather than in two distinct lateral centers placed one beside the other, it is evident that this horn core is in reality morphologically quite distinct from the nasals. In this respect the nasal horn cores differ greatly horn the supraorbital horn cores, which are simple out- growths from the postfrontals, and therefore are morphologically* a part of their supporting elements. Unlike the supraorbital horn cores, the nasal horn cores are not hollow at the base, but consist throughout of loose cellular bone. Like the supraorbital horn cores, the external surface Of the nasal horn core is marked by numerous vascular impressions, showing that in life they were insheathed with horn. Morphologically the nasal horn cores may be considered as dermal or epidermal ossifications similar to the epijugals, epoccipitals, the rostral, and the predentary, and as quite distinct from the frontal horn cores. In the collection of the Walker Museum of the University of Chicago there is a detached nasal horn core (No. 544) pertaining to a young individual. The specimen, which apparently belonged to Triceratops prorsus, although of a different form from the nasal horn of the type of that species, is in a splendid state of preservation, and shows well the sutural surfaces through which it was attached to the nasals with which, had the animal lived, it would later have become coossified. This horn core is compressed supero-inferiorly, so that its transverse diameter exceeds its vertical. Its principal characters are well shown in fig. 29, which I am able to introduce here through the kindness of Dr. S. W. Williston. The variation in form and size of the nasal a
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