O. C. Marsh— Skull of the Gigantic Ceratopsidm. 503 



The front part of the skull shows a very high degree of 

 specialization, and the lower jaws have been modified in con- 

 nection with it. In front of the premaxillaries, there is a large 

 massive bone, not before seen in any vertebrate, which has been 

 called by the writer, the rostral bone (os rostrale). It covers 

 the anterior margins of the premaxillaries, and its sharp 

 inferior edge is continuous with their lower border. This 

 bone is much compressed, and its surface very rugose, showing 

 that it was covered with a strong horny beak. It is a dermal 

 ossification, and corresponds to the pre-dentary bone below. 

 The latter, in this genus, is also sharp and rugose, and likewise 

 was protected by a strong horny covering. The two together 

 closely resemble the beak of some of the turtles, and as a 

 whole must have formed a most powerful weapon of offense. 



In the skull figured on Plate XII, the rostral bone was free, 

 and was not secured. This was also true of the pre-dentary 

 bone, and the nasal horn-core. Hence these parts are repre- 

 sented in outline, taken from another specimen, in which they 

 are all present, and in good preservation. 



The pre maxillary bones are large, and much compressed 

 transversely. Their inner surfaces are flat, and meet each 

 other closely on the median line. In old specimens, they are 

 firmly coossified with each other, and with the rostral bone. 

 They send upward a strong process to support the massive 

 nasals. Another process, long and slender, extends upward 

 and backward, forming a suture with the maxillary behind, 

 and uniting in front with a descending branch of the nasal. 

 The premaxillaries are much excavated externally for the 

 narial aperture, and form its lower margin. They are entirely 

 edentulous. 



The maxillaries are thick, massive bones of moderate size, 

 and subtriangular in outline when seen from the side. Their 

 front margin is bounded mainly by the premaxillaries. They 

 meet the prefrontal and lachrymal above, and also the jngal. 

 The alveolar border is narrow, and the teeth small, with only a 

 single row in use at the same time. The teeth resemble, in 

 general form, those of Hadrosaurus. 



The nasal bones are large and massive, and greatly thickened 

 anteriorly to support the nasal horn-core. In the skull figured 

 on Plate XII, these bones are separate, but in older individuals, 

 they are firmly coossified with each other, and with the 

 frontals. The nasal horn-core ossifies from a separate centre, 

 but in adult animals, it unites closely with the nasals, all traces 

 of the connection being lost. It varies much in form in 

 different species. 



