HOLLAND : THE OSTEOLOGY OF DIPLODOCUS MARSH 247 
edly that of an older specimen, in which the different elements have become thor- 
oughly fused and codssified. The elements of the atlas represented in the specimen 
belonging to the American Museum of Natural History are an intercentrum, two 
neural arches, and the odontoid process, figures of which, through the kindness of 
Dr. H. F. Osborn, I am permitted to give. 
Fre. 11. Posterior view of intercentrum of atlas. a, right neurapophysial facet; 0, left neurapophysial facet ; 
o, odontoid facet. (One half natural size. ) 
Fic. 12. Anterior view of intercentrum of atlas. a, right neuraphysial facet ; 0, left neuraphysial facet ; 0.c., 
occipito-condylar facet. (One half natural size. ) 
Fia. 13. Inferior view of intercentrum of atlas. (One half natural size. ) 
The Intercentrum (Figs. 11-15). — The intercentrum is an irregularly oblong bone, 
flattened and somewhat concave on its lower side, sending forward a curving lip-lke 
anterior projection along its lower anterior margin, and on either side of the hind 
margin throwing out two articular processes. The anterior surface is concave, 
adapted to accommodate the occipital condyle. The posterior surface is also con- 
cave, adapted to receive the odontoid process. On either side are broad facets for 
the right and left neural arches. The characteristic features of this bone are shown 
in the accompanying illustrations (Figs. 11, 12, and 13). 
The Neural Arches (Figs. 14 and 15).— The neural arches are irregular bones, 
strongly concave inwardly and convex outwardly. They articulate with the inter- 
14. 
\ 
Fic. 14. External view of left neural arch of atlas. p, pedicle; 7, lamina; s.p., spinous process; p.z., post- 
zygapophysis. (One half natural size. ) 
Fic. 15. Internal view of the right neural arch of atlas. jp, pedicle ; s.p., spinous process ; p.z., postzygapoph- 
ysis. (One half natural size.) 
centrum by means of a strongly-developed pedicle which flares out on all sides at 
the point where it codssifies with the intercentrum. In front the bone sends in- 
