HOLLAND : THE OSTEOLOGY OF DIPLODOCUS MARSH 249 
much distorted as to make it impossible to understand its relation to the atlas and 
axis. The bone may be described as having, roughly speaking, the form of a 
quarter of a sphere, the rounded surface fitting into the odontoid notch of the in- 
tercentrum of the atlas. This surface is not, however, perfectly rounded, but is 
somewhat constricted about the middle. The upper surface is approximately flat 
horizontally. Running through the middle is a small groove or sulcus, narrow in 
front and widening behind. The posterior surface is slightly concave, with two 
small rugosities projecting backwards, one on either side of the sulcus which has 
been described as running through the upper surface. The form of the bone is 
best understood by reference to the accompanying figures (Figs. 16-19). The 
superior surface of the odontoid of course formed, when in place, a portion of the 
lower wall of the spinal canal. 
The Aais (Figs. 20-22). — 'The axis of specimen No. 84 (Carnegie Musum Cata- 
logue of Vertebrate Fossils) has been very carefully described and has been well 
represented by Mr. Hatcher. He says in his description: ‘A short cervical rib 
without anterior process springs from the side of the centrum near its inferior mar- 
gin and anterior extremity.” A very careful reéxamination by the writer of the 
Fic. 20. Supposed rib of the atlas of Diplodocus preserved in the American Museum of Natural History. The 
upper figure is an external view of the rib, the middle figure is an internal view, the lower figure is an inferior view. 
specimen upon which Mr. Hatcher's description was based leads him to think that 
the cervical rib on both sides has sustained injury, and that only a portion remains 
adherent to the centrum. Accompanying the elements of the atlas sent to the 
writer for study by the kindness of Professor Osborn are two bones, undoubtedly 
cervical ribs. ‘They are both bones belonging on the right side of the centra. They 
are reported to have been found at the same place at which the atlas was found. 
The writer is inclined to think that the larger of these two bones (Fig. 20), was prob- 
ably the rib of the atlas and indeed it requires but little effort to see that it might 
very well have served such a function, and that the smaller bone (Fig. 21) was the 
