THE OSTEOLOGY OF ELOTHERHJM. 29/ 



this dimension exceeding the antero-posterior length, and is proportionately much greater 

 than in Hippopotamus or the modern snillines. 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 very 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 ventral 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 Motherium, 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 8us the sternum is still more like that of Elotherimti, but has a decidedly 

 longer and lower presternum. 



VI. The Fore Limb. 



The fore limb of Elotherium is quite elongate and, in connection with the shallow 

 thorax, and very long neural spines of the anterior thoracic vertebrae, it gives to the 

 skeleton a somewhat stilted appearance. 



