HATCHER : DIPLODOCUS (mARSH) • 19 



gives any description of this vertebra. His description of the entire vertebral 

 column of Diplodocus in his "Dinosaurs of North America" is quite brief consider- 

 ing the highly interesting and remarkably specialized nature of this part of the 

 skeleton. He dispenses with the vertebral column in two short paragraphs of eleven 

 lines. From Mar'sh's figures the atlas appears to be rather short and very narrow, 

 without expanded transverse processes. The neural spine is absent or very low, with 

 a short anterior projection and a rather long posterior projection, with small posterior 

 zygapophysial articular surfaces. There is a marked constriction between the neural 

 canal and the cavity for the odontoid process. A small cervical rib is seen on the 

 lower margin of either side of the posterior extremity. 



The Axis. — The axis in this skeleton is quite complete and remarkably symmetri- 

 cal for a vertebra of Diplodocus. The greatest length of the centrum is a little less 



Fig. 4. Atlas of Diplodocus longus ; side view ; one half natui'al size, a, articular face for axis ; c, 

 cup ; r, face for rib ; 2, posterior zygapophyses. After Marsh. 



Fig. 5. Atlas oi Diplodocus longus; front view ; one half natural size ; c, cup ; 0, cavity for odontoid 

 process ; I, neural canal. After Marsh. 



than the distance from its ventral surface to the top of the neural spine. Promi- 

 nent postzygapophysial laminse spring from the posterior and superior border of 

 the neural arch. These diverge and extend upward and backward until they reach 

 the summits of the postzygapophyses, when they are directed suddenly forwards 

 and rapidly converge, meeting anteriorly and superiorly in the middle line to form 

 the prespinal lamina. They thus enclose a deep postspinal cavity which opens pos- 

 teriorly and externally. The anterior zygapophyses are very small and low. They 

 occupy expansions projecting from near the middle of the sides of the neural arch, 

 do not project forward beyond its anterior border, and look directly upward. The 

 posterior zygapophyses are much elevated and face outward, downward and back- 



