NO. 1600. OSTEOLOGY OF CAMPTOSAURUS—GILMORE. 221 



ramus, which, from its position, I have identified as the prearticular. 

 jMarsh considered tliis bone the articular, as is indicated b}^ the abbre- 

 viation "art." still remaining on the specimen and jilainl}'^ shown in 

 fig. 1, Plate 0, reproduced here from a photograph. This inter- 

 pretation, however, leaves the small element on the supero-posterior 

 l)art of the jaw without designation. That these two bcmes are dis- 

 tinct elements there can be no (question, as all of the sutures in this 

 specimen are clearly defined, and, moreover, the posterior part of the 

 ramus of No. 1880, Yale Museum, shows the articular in position 

 while the prearticular has been displaced and is missing. 



Dollo" considers an element occupying a similar position in the 

 jaw of I guanodon hemissm'tensis the surangular. I am inclined to 

 believe that this element represents the prearticular and that the 

 surangular is on the supero-external part of the posterior end of the 

 ramus, as in Gamytosaurus^ but which he indicates as the articular. 

 The pi-esence of an external mandibular foramen occupying rela- 

 tively the same position as found in the surangular of Camptosaicnis 

 (see fig. 2, sf) is also suggestive of the correctness of this inter- 

 pretation. 



In Camftomurus the prearticular is an elongate bone lying dorsal 

 to the supero-internal border of the angular, and extending nearly 

 if not fully to the posterior termination of the jaw. Anteriorly its 

 forward extremity is covered by the overlying splenial. Its upper 

 posterior border is concave and, with the surangular and articular, 

 forms a cotyloid surface for the quadrate. In comparison wdth the 

 size of the end of the quadrate this surface is capacious, an arrange- 

 ment which Avould have permitted of free movement of the jaws 

 upon the quadrate. On the internal median surface, just before this 

 element disappears under the splenial, may be seen an elongated oval 

 foramen, which probably represents the internal mandibular foramen 

 of most reptiles (see /, Plate 9, fig. 2). I do not know that this 

 element has been observed before in a member of the orthopodous 

 dinosaurs, although it is present in most turtles and members of the 

 Pelycosauria, and it would appear to indicate a primitive arrange- 

 ment of the elements of the mandible.'' 



Splenial. — The- splenial is a comparatively thin, flattened bone 

 applied to the inner surface of the ramus. On the lower posterior 



« Dollo, Bull. Bruxelles Mus. Roy. d'Hist. Nat. de Belgique, II, 1883, pi. ix, 

 fig. 3. 



* Wllliston gives au interesting discussion of this element in his description 

 of Dolicliorhj/nchoiJS (Field Columbian Mus. Pub. No. 73, Geol. Ser., II, No. 1, 

 1903, pp. 29 to 32). He calls attention to the fact that Baur (Amer. Nat., 1891) 

 believes that the element usually considered the articular is composed of two 

 bones in the young Sphenodon and the conditions found in the jaws similar to 

 those of the Testudinata. Baur assumes these elements to be present in all 

 reptilian mandibles, but in the adult skull their identity becomes obliterated by 

 the ankylosis of the suture. 



