294 THE OSTEOLOGY OF ELOTHERIUM. 
to much 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 eyersion of the anterior ends of the 
ilia, but even here they are weak. 
The trunk-vertebre of Hippopotamus are much more massively constructed than 
those of Hlotheriwm, the decrease in length of the thoracic spines posteriorly is more 
gradual, while the neural spines 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 Hlotheriwm, but in the former the transverse processes of the thoracic 
vertebree are perforated by vertical canals, and those of the lumbars are much longer and 
stouter. 
The sacrum consists of two vertebree only. The first has a broad, depressed centrum 
and yery large pleurapophyses, which carry most of the weight of the ila, though the 
second sacral has also a limited contact with the pelvis. On the first vertebra the 
prezygapophyses are very well-developed and haye large episphenial processes to receive 
those of the last lumbar. The two neural spines are co(ssified into a high but short 
ridge. The second sacral has a yery much smaller and especially a narrower centrum 
than the first, and retains moderately complete postzygapophyses. 
In Hippopotamus and in Sus the sacrum is relatively much larger than in Hlotheriwm, 
and consists of at least four vertebrae, sometimes eyen as many as six. Hyven in aged 
individuals of the White River genus I haye not seen more than two vertebre in the 
sacrum. ; ; 
The caudal vertebre (Pl. 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 conyex 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 transyerse 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 hemapophyses. 
The succeeding caudal vertebrae resemble the first 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 hemapophyses, on the 
