38 



THE CERATOPSIA 



the conjectural position of the pair of hyoids relative to the jaws, it will be seen that the bones 

 enclose a space within which the vocal apparatus may have been situated. If so, the latter must 

 have been very large, implying on the part of these animals a voice commensurate with their bulk 

 and — judging from their armament — general aggressiveness. 



The curved body of the bone, which is unlike any other that I have seen, is provisionally referred 

 to the basihyal and may constitute half of the first branchial arch. 



Fig. 6. — Hyoid of Triceratop serratus, No. 1823 Y.P.M., y 2 natural size, d, dentary; h, hyoid; a, anterior; 



f, posterior. 



Compared with the so-called thyrohyals of Centrosaurus, the two elements have nothing in 

 common other than being in each instance portions of the hyoid complex which can be known in its 

 entirety only through future discovery. 



Hyoid elements in the skull of Centrosaurus afertus No. 4519 R.O.M. have been described 

 by Parks 8 as follows: "Both these elements were discovered in the present specimen with their 

 anterior ends touching the internal surface of the maxillaries near their posterior extremities, and 

 the posterior ends almost meeting in the midline. The bones are long and slender, the right 

 measuring 168 mm. and the left (slightly broken anteriorly) 162 mm. in length. The anterior end 

 is somewhat expanded, the upper surface flattish and the posterior end inflected slightly outwards. 

 The under surface is more convex in its anterior portion and this convexity rises into a distinct ridge 

 near the outer side of the bone less than half way down. This ridge crosses the bone diagonally and 

 terminates on the inner edge at the posterior extremity, thus giving a twisted appearance to the bone 

 when viewed from the ventral side. The width at midlength is 1 6 to 18 mm. and the thickness 

 (dorso-ventral) about 12 mm." These bones are comparable to the thyrohyal preserved in a Camp- 

 tosaurus skull, No. 1887 Y.P.M. 9 The chief difference seems to lie in a decided curvature in the 

 Camptosaurus bone. Doubtless these bones were homologous elements ; they cannot have been the 

 same as the peculiar hyoid of Triceratops. 



8 Parks, W. A., 1921, p. 57. 



9 Gilmore, C. W., 1909, p. 224, PL 9. 



