256 MEMOIRS OF THE CARNEGIE MUSEUM. 
Exit for the first pair of spinal nerves is furnished by the large foramen located 
above on either side just behind the point where the inner margin of the condylar 
surface joins the superior arch. This foramen is succeeded externally by a deep 
groove, which passes forward and downward through the notch between the anterior 
lip-like projection of the anterior articulating surface and the anterior prolongation 
of the lateral process, and then may in some specimens be traced backward on the 
under surface of the transverse process to the deep fossa which lies on either side 
before the opening of the vertebrarterial canal. 
The vertebrarterial canal passes through the posterior half of the transverse 
process along the line of union with the body of the bone, emerging behind at the 
point where the posterior articulating portion of the atlas forms a notch with the 
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Fra. 23. Atlas of Moropus elatus Marsh (C. M. Cat. No. 1604). 4%. 1. Posterior view. 2. Anterior view. 
3. Superior view. 4. Inferior view. 
posterior extremity of the transverse process. This arrangement of the foramen 
transversarium, so far as the writer has been able to ascertain, differs from that 
found in many recent mammalia, where the vertebrarterial canal is represented in 
the atlas by a foramen piercing the ale about the middle, but is precisely that which 
occurs in the case of the Felide, the Tapiride, and Camelide, and in Entelodon, 
Trigonias, Dinoceras, and the Titanotheres among extinct forms. In almost all 
specimens there appears on either side below the transverse process and midway 
between the anterior notch and the anterior opening of the vertebrarterial canal a 
