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 every 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 from 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 and supraorbital horn cores in some of the 

 various genera and species of the Ceratopsia is well shown in Pis. IV and Y, where similar 

 views of a number of such, drawn to the same scale, are reproduced for comparison. 



Fig. 29. — A, Nasal horn core (No. 544, University of Chi- 

 cago collection) of young specimen of Triceratops 

 prorsus as seen from right side; B, proximal end of 

 same, showing sutural surface for nasal. One-fourth 

 natural size. 



A B C 



Fig. 30.— Rostral of type of Triceratops elatus, No. 2116, U. S. National Museum. A, As seen from left side; B, front view; C, inferior 

 view, a, Anterior end; s. superior process; i, inferior process. After Marsh. 



THE ROSTRAL. 



This element embraces the anterior extremity of the premaxillaries, and becomes early 

 in the life of the individual firmly united with them. Anteriorly it presents the appearance 

 of a sharp cutting beak, while posteriorly it is triradiate, as shown in C, fig. 30. The median 

 and superior of the three branches is keeled beneath, the keel being wedged in between the 

 anterior extremities of the premaxillaries, which are embraced laterally by rugose folds from 

 the superior and external borders of the same process or branch of the rostral. Inferiorly 

 and laterally the rostral sends backward two rather slender processes, which embrace the 



MON XLIX — 07 3 



