294 THE OSTEOLOGY OF ELOTHEBIUM. 



to mueli individual variation. Metapophyses are prominent only on the first and second 

 lumbars, rudimentary on the third and absent from the others. The transverse processes 

 are very feebly developed in proportion to the size of the vertebrae. On the first lumbar 

 they are short and straight, and gradually increase in length up to the fifth, but in all 

 they are strikingly thin and slender. The last lumbar has transverse processes of unusual 

 length, space for them being obtained by the sudden eversion of the anterior ends of the 

 ilia, but even here they are weak. 



The trunk-vertebra? of Hippopotamus are much more massively constructed than 

 those of Elotherium, the decrease in length of the thoracic spines posteriorly is more 

 gradual, while the neural sj>ines and transverse processes of the lumbars are much longer 

 and in every way heavier. The thoraco-lumbar series of Sus bears considerable resem- 

 blance to that of Elotherium, but in the former the transverse processes of the thoracic 

 vertebra? are perforated by vertical canals, and those of the lumbars are much longer and 

 stouter. 



The sacrum consists of two vertebra? only. The first has a broad, depressed centrum 

 and very large pleurapophyses, which carry most of the weight of the ilia, though the 

 second sacral lias also a limited contact with the pelvis. On the first vertebra the 

 prezygapophyses are very well-developed and have large episphenial processes to receive 

 those of the last lumbar. The two neural spines are cofssified into a high but short 

 ridge. The second sacral has a very much smaller and especially a narrower centrum 

 than the first, and retains moderately complete postzygapophyses. 



In Hippopotamus and in Sm the sacrum is relatively much larger than in Elotherhim, 

 and consists of at least four vertebrae, sometimes even as many as six. Even in aged 

 individuals of the White River genus I have not seen more than two vertebra? in the 

 sacrum. 



The caudal vertebra} (PI. XVIII, Figs. 7, 8, 9), of which fifteen are preserved in 

 association with one individual, indicate- a tail of only moderate length, and present a 

 number of peculiarities. The first caudal has somewhat the appearance of a miniature 

 lumbar; its centrum is short, broad and depressed, with quite strongly convex faces; the 

 neural canal is relatively large and a distinct, though small, neural spine is present. 

 The zygapophyses, especially the anterior pair, are large and prominent and project 

 much in front of and behind the centrum. The transverse processes are quite long and 

 heavy, and are directed outward and backward. A pair of tubercles on the ventral side 

 of the centrum represent rudimentary hsemapophyses. 



The succeeding caudal vertebra? resemble the Hist in a general way, but passing 

 backward, the centra become more and more slender and elongate, while the neural canal 

 diminishes in size, and the various processes are reduced. The ha?mapophys< s, on the 



