46 



BULLETIN 89, UNITED STATES NATIONAL MUSEUM. 



The neuracentra are rather irregularly shaped pieces which form the sides 

 of the neural arch. The articular end which rests upon the intercentruru is expanded 

 antero-posteriorly, having two faces which meet at an obtuse angle. The poste- 

 rior face rests upon the end of the intercentrum while the anterior and smaller one 

 of the two looks inward and forward and contributes to the formation of the cup 

 for articulation with the occipital condyle. In the collections of the National 

 Museum there are four individuals (Nos. 4934, 4935, 6645, and 7725) which have 

 the atlas preserved. The first three have the neuracentra detached from the 

 intercentrum, but the latter, from which figure 13 was drawn, shows these pieces 

 firmly coossified. Although the upper part of these arches is incomplete (having 



been restored from the other spe- 

 cimens) this specimen gives a 

 most accurate idea of the manner 

 of the articulation of these lateral 

 pieces in the atlas of Stegosaurus, 

 as is clearly shown in figure 13, 

 A. Just above the neck-like con- 

 traction of the pedicel the neu- 

 racentrum gives off two processes, 

 one directed inward and upward, 

 which with the corresponding 

 process of the opposite side forms 

 the covering of the upper part of 

 the neural canal. The neuracen- 

 tra did not unite on the median 

 line but were probably bound to- 

 gether by ligaments. The post- 

 zygapophysial process is more 

 slender than the one just described 

 and is directed upward and backward, its lower internal surface being adapted 

 for articulation with the prezygapophysis of the axis. 



The odontoid is a small bone (fig. 14, o), the upper surface, which is slightly 

 concave transversely, forming the floor of the neural canal. The whole lower 

 part of the bone is rounded transversely. When articulated the anterior part is 

 in contact with the upper concave surface of the intercentrum of the atlas. In 

 position the smooth anterior end abuts against the posterior end of the occipital 

 condyle. The posterior end is slightly concave and is closely applied and in some 

 instances (see axis of No. 4935, fig. 14) is partly coossified with the centrum of the 



axis. 



Measurements of atlases. 



Fig. 13.— Atlas of Stegosaurus stenops ? Marsh. Cat. no. 6645, 

 U.S.N.M J Nat. size. A, Viewed from left side; in, inter- 

 centrum; n, neuracentrum; s, neural spine,- z', posterior 



ZYGAPOPHYSIS; RESTORED FROM THE ATLAS OF NO. 4935, U.S.N. M. 



B, Viewed from the front; a, cup or articulating surface 



FOR THE OCCIPITAL CONDYLE OF THE SEULL," S, NEURAL PROCESSES. 



C, Viewed from the back; p, broken surfaces whence post- 



ZYGAPHYSIAL PROCESS IS GIVEN OFF; T , FACETS FOR CERVICAL EIBS. 





No. 4934. 



No. 4935. 



No. 6645. 





mm. 

 22 

 55 



mm. . 

 24 

 61 



mm. 

 24 

 61 







