THE OSTEOLOGY OF ELOTHERIUM. 297 
this dimension exceeding the antero-posterior length, and is proportionately much greater 
than in Hippopotamus or the modern suillines. The body of this segment is extremely 
thin, but the anterior border, and to some extent the ventral border also, is thickened and 
rugose. ‘The facets for the first pair of sternal ribs form prominences, which are situated 
near together and close to the postero-superior angles of the segment, so that nearly the 
entire length of the latter projects in front of the first pair of ribs. 
Of the mesosternum four segments and a part of the fifth are preserved. The first 
segment somewhat resembles the presternum in shape, being short, narrow and yery deep ; 
the dorsal border is much thicker and wider than any other part of the segment, and the 
ventral border is also thickened, though in a less marked degree. Posteriorly, this 
element becomes somewhat wider and shallower. The second segment of the mesosternum 
is decidedly broader and shallower than the first, but still retains a very unusual 
degree of vertical depth. Both the dorsal and ventral surfaces are much broadened, 
while the body of the bone is a thin, vertical plate, which connects the horizontally 
directed dorsal and ventral borders, giving a cross-section somewhat like that of an 
I-beam. In the third segment these progressive changes are carried still farther, and the 
bone becomes very distinctly broader and lower than the second segment. The dorsal 
and yentral borders still project much beyond the vertical connecting plate ; this plate, 
however, is much thicker transversely than in the preceding segment. The ventral 
surface is rendered quite strongly concave by- the elevation of its lateral borders. In 
part, this concavity may be due to the pressure which has somewhat distorted the entire 
sternum, but the ventral groove is so symmetrical that it can hardly be altogether due to 
distortion. The fourth and fifth segments exhibit similar changes, each one being 
broader and lower than the one in front of it; the vertical plate becomes very much 
thicker and the ventral groove more widely open. Though the specimen is of an animal 
past maturity, yet the last three segments distinctly show the median. suture, along which 
their lateral halves united. 
In Hippopotamus the breast-bone is quite like that of H/otherium, but the presternum 
is longer and not of such exaggerated depth, and the rib-facets are placed much nearer to 
the anterior end, while the mesosternum consists of fewer, broader and shallower 
segments. In Sus the sternum is still more like that of Hlotheriwm, but has a decidedly 
longer and lower presternum. 
VI. Tue Fore Limes. 
The fore limb of Elotheriwm is quite elongate and, in connection with the shallow 
thorax, and very long neural spines of the anterior thoracic yertebre, it gives to the 
skeleton a somewhat stilted appearance. 
