Peterson: The Osteology of Promerycochcerus. 171 



The First Dorsal (Fig. ii). — The most conspicuous part of the first 

 dorsal is the enormously large neural sijine, the heaviest in the vertebral 

 column. The centrum of this vertebra is depressed and in general 

 similar to that of the seventh cervical. The inferior surface of the 

 centrum is not so broad behind, but the anterior articulation is nearly 

 as hemispherical as in the vertebra preceding it. The capitular and 

 tubercular facets are large and separated only by a sharp bony ridge 

 on the under surface near the base of the heavy transverse process. 

 The posterior intervertebral notch is deep, and is continued as a deep 

 groove back of the capitular facet for the second rib, terminating 

 suddenly on the rugose inferior lateral face of the centrum. The 

 large neural spine has the transverse diameter of the posterior border 

 comparatively greater and more roughened for muscular attachments 

 than in the Oligocene genus. 



The Second Dorsal (Fig. 12). — The centrum of the second dorsal is 

 much less depressed than that of the first, and the keel begins to 

 become sharp as is the case further back in the thorax of Merycoidodon 

 cidbertsoni. In the latter genus the keel of the first, second, third, 

 and fourth dorsals is broader and on the anterior part of the ventral 

 face there are two keels, one on either side of the median line which is 

 faintly indicated. On the inferior lateral surface of the centrum of the 

 second dorsal in Promerycochcerus carrikeri there is a round deep pit, 

 from which a deep groove leads in a supero-lateral direction, curving 

 backward at the base of the transverse process behind the capitular 

 facet to the posterior intervertebral notch, as in the preceding vertebra. 

 The capitular facets are larger and deeper, but the transverse process 

 is not so large as in the first dorsal. The second dorsal has the highest 

 neural spine in the vertebral column, but its antero-posterior diameter 

 and width are less than those of the first dorsal vertebra. 



The Third Dorsal (Fig. 13).— This vertebra differs chiefly from the 

 one before it in having a smaller and more backwardly inclined neural 

 spine. The transverse process is also smaller. Otherwise there are 

 no differences of importance between these two vertebrse. 



The Fourth, Fifth, and Sixth Dorsals (Fig. 14). — These vertebra? 

 are so similar that the description of one suffices for all. The centra 

 gradually increase in length and weight, while they decrease in width. 

 The ventral and lateral surfaces increase in convexity fore-and-aft, 

 and the transverse processes and neural spines decrease in size. The 

 summits of the spines are enlarged into rounded rugose knobs, in 

 which the antero-posterior exceeds the transverse diameter. 



