PETERSON: THE AMERICAN DICERATHERES. 



439 



the first dorsal spine is high, broad, and rugose, as shown in Fig. 22, while in 

 many specimens, fully adult and old, this spine is 50 mm. shorter, and sometimes 

 even more. In the anterior dorsals the curvature of the neural spine also varies 

 from a comparatively straight spine to one with a gentle sigmoid curve. The 

 latter are generally those with the longer and heavier spines. The neural spine of 

 the second dorsal is suddenly reduced in size, but back of the second the reduction 

 is more gradual. The anterior dorsals have short, broad, and depressed centra, 

 while further back they are higher, narrower, and terminate ventrally in better 

 defined keels. The intervertebral notch is deep and in the posterior upper side 

 of the centrum it continues downward in a broad and well-defined groove, princi- 

 pally due to the greatly elevated border of the capitular facet on the centrum. 



Fig. 23. Fig. 24. 



Fig. 23. Diceratherium cooki Peterson. No. 2499, Coll. Carnegie Museum. Lateral and posterior 



views of tenth dorsal. X i- 

 Fig. 24. Diceratherium cooki Peterson. No. 2470a, Coll. Carnegie Museum. Lateral and posterior 



views of eighteenth dorsal. X i. 



In the neighborhood of the eighth, ninth, and tenth dorsals there is usually a fora- 

 men formed at this notch, which is characteristic of all posterior dorsals except 

 the last. (See Figs. 23-24.) The fourteenth and fifteenth dorsals have the neural 

 spines broader and more lumbar-like; the mammillary processes, so characteristic 

 of the transverse processes of the dorsals are also longer and project forward in 

 these vertebrae to a greater extent. The inferior aspect of the centra vary from a 

 gently rounded to a more decided ventral keel, possibly due to sex. 



Lumbar Vertebrce. Figs. 25-27. There are five lumbar vertebrae. This 

 causes this part of the spinal column to be rather short. In general outline the 



