OSTEOLOGY OF THE AEMOKED DINOSAUEIA. 



37 



Lower Jaw. 

 Plates 5, 7, 8, and 9. 



The mandible iii Stegosaurus consists of 13 elements, and if the coronoid 

 is present there are 15. The dentary, angular, surangular, articular, pre- 

 articular, and splenial are paired bones, the predentary being single and articulating 

 with the ends of both rami. The coronoid, a paired element, is probably present, 

 but its presence or absence can not be determined from the available specimens. 



The following description of the several elements of the lower mandible is to 

 a great extent based upon the well-preserved jaws of S. stenops, No. 4934. It 

 might be well, however, to explain that the jaws were found articulated with the 

 skull, to which they are so closely applied that it has been deemed unwise to attempt 

 their separation. 



Dentary (d.). — The dentary, as in all dinosaurs, is the largest bone of the man- 

 dible. In Stegosaurus it is especially remarkable on account of the extreme trans- 

 verse thinness of the lower half and the wide shelf of bone exterior to the row of 

 teeth on the dorsal border. These features are best shown by the portion of the 

 right dentary pertaining to specimen No. 4935, shown in figure 8. The transverse 

 width of the median 

 lower border of this 

 bone is 7 mm.; the 

 same measurement 

 of the dental border 

 is 21 mm. The ex- 

 tern al surf ac e is c om- 

 paratively smooth 

 and flat, and the 

 vertical diameter 

 decreases gradually 

 from back toward 



the front. Below the anterior border of the orbit, in specimen No. 4934, it has a 

 depth of 68 mm.; below the posterior border of the nares a depth of 41 mm. 



The anterior end curves downward (fig. 8) and inward, the two dentaries 

 forming a "spoutlike" symphysial end as in Camptosaurus. 



The dorsal border is comparatively wide, the tooth row being placed on the 

 extreme internal border. Along its external border, beginning opposite the first 

 tooth, a ridge gradually rises from this surface until opposite the last tooth it 

 attains a height of 17 mm. above the border of the alveolus. Between this ridge 

 and the dental series is a row of foramina, which evidently correspond to those on 

 the external surface of the Camptosaurus dentary, and which doubtless served for 

 the transmission of nerves and nutrient blood vessels to the lips. These foramina 

 lead to a canal traversing the length of the heavier part of the bone and which opens 

 anteriorly near the symphysis. This is the mental foramen through which a branch 

 of the fifth nerve emerges. 



The anterior part of the dorsal border where it turns down to meet the pre- 

 dentary, is shallow though broadly grooved. On the internal side of the dentary 

 of No. 4935 (fig. 8) alveoli for 23 teeth can be counted. These occupy a space 

 51873°— BuU. 83—14 i 



Fig. 8. — Anterior elilf of right dentary of Stegosaurus sp. Cat. no. 4935, U.S.N.M. 

 J Nat. Size. Internal vte-w. a, Anterior end; p, posterior end; s, symphtsul 

 SURFACE. Shows the complete dental series of the lower mw as consisting of 

 23 functional teeth. 



