OSTEOLOGY OF THE ARMOKED DINOSATJBIA. 39 



the posterior end the upper border is deeply cut out, only a narrow part of the 

 bone extending posterior to the articular end of the quadrate in the articulated 

 jaw. (See sa., pi. 5.) Internally it laps along the articular and also the prearticular. 

 It does not contribute to the formation of the cotylus of the lower jaw, as in Camp- 

 tosaurus. The external mandibular foramen found in the surangular of Camptosau- 

 rus, Iguanodon, and Triceratops does not appear to be present in this genus. 1 



Articular (ar.) . — The articular is a small block-like bone that forms the greater 

 portion of the articular surface for the quadrate. It is embraced below by the 

 prearticular, while on its external surface it is lapped by the angular and surangular. 

 In specimen No. 4934, upon which the present description is based, this element 

 has a transverse width of 35 mm. On the median posterior border it has a vertical 

 depth of 15 mm. On account of the articulating quadrate bones the anterior 

 extent of this element can not be determined. The manner in which the angular, 

 surangular, prearticular, and articular unite is well shown in plates 7, 8, and in 

 plate 9, figure 2, which show all of these bones in their normal relations to one 

 another. 



Prearticular (p. ar.). — Marsh in his representations of the lower jaw of Stego- 

 saurus, did not recognize the presence of the prearticular but called the combined 

 elements 2 the articular (ar.). Since this element has recently been found in the 

 jaws of other members of the Predentata, i. e., in Camptosaurus by Gilmore 3 and 

 in Triceratops by von Huene 4 it was not surprising to find it existing as a sepa- 

 rate element in Stegosaurus. An examination of the jaws of No. 4934 showed 

 clearly on the posterior ends of both rami the presence of a suture between the 

 articular and an element which embraced it on the inferior surface, and which is 

 identified as the prearticular 5 (p. ar., pis. 7 and 8). 



In Stegosaurus the prearticular is a moderately long bone, lying internal to the 

 angular which underlaps it for some distance on its median inferior border. The 

 anterior end of the prearticular in the articulated jaw disappears under the overlying 

 splenial. 



The anterior half of this element exists as a thin vertical plate of bone applied 

 to the inner side of the jaw, but the posterior portion changes from the vertical to 

 a horizontal plane, gradually widening until its maximum width is reached, where it 

 underlaps the articular. 



The truncated posterior end terminates slightly forward of the posterior ex- 

 tremity of the articular. A small portion of the posterior edge of this bone is 

 visible from a side view, just below the angular bone (p. ar., pi. 5). 



Coronoid (c). — As mentioned previously on account of the close articulation of 

 the jaws with the skull in specimen No. 4934, the presence of this element can not 

 be positively determined. 



' Tn the English Translation of Zittel's Text-Book on Paleontology, vol. 2, 1302, p. 211, it is said to be present. 

 = Amer. Joum. Sci., ser. 3, vol. 34, 18S-7, fig. 1, pi. 6. 

 3 Proc. U. S. Nat. Mus., vol. 36, 1909, pp. 220-221. 

 « Neues .Takrhuch, Jahrg. 1911 pt. 2. 1912, pp. 160-161 . * 



3 Since the above was written Huene in an article in Neues Jahrbuch, vol. 37, 1914, p. 581, recognizes and describes a 

 prearticular in the lower jaw of Stegosaurus. 



