OSTEOLOGY OF THE HYOPOTAMLDiE. 
81 
as in the corresponding hone of Diplopus (Plate XXXVIII. fig. 6, hi.), this projection is 
much more inclined (perhaps 30° with the horizon). The reason of this difference is given 
by the unciform, as the radial inferior truncature of this bone (Plate XXXVIII. fig. 5, u), 
to which the projection of the third metacarpal is articulated in Hyopotamus, becomes 
nearly vertical, and helps to constitute the inner radial wall of the same bone in Diplopus 
(Plate XXXVIII. fig. 8). 
The proximal surface of the third metacarpal (fig. 5, hi.) has an elongated, somewhat 
triangular, flat facet for the os magnum ; this upper surface is sloping inwards, so as to 
allow the second metacarpal to lean laterally on this margin, and reach its facet on the 
os magnum, as seen in fig. 5. In Diplopus, on the contrary (fig. 6, hi.), this upper 
radial margin of the proximal surface is produced upwards (as in Dalceoclmrus , Plate 
XXXVII. fig. 6) ; and therefore I think that, in the didactyle form, the rudiment of the 
second digit could not reach as high as the os magnum. 
The ulnar side of the third metacarpal has an anterior reniform and a posterior oval 
facet ; both articulate with corresponding facets on the radial side of the fourth digit. 
In the didactyle Diplopus , this anterior reniform facet is excavated into a deep hole for 
a corresponding large projection of the fourth metacarpal, as seen in Plate XXXVIII. 
fig. 6 : thus the interlocking of the two middle digits is much closer in the didactyle genus ; 
and this is quite natural, as the foot, having no lateral digits, required a firmer structure. 
The radial side of the third metacarpal in Hyopotamus has on its anterior part a tole- 
rably long (7 millims.) facet, which is uninterruptedly united with the somewhat sloping- 
radial margin of the proximal surface. This facet is destined for the second metacarpal, 
which has a corresponding surface on its inner (ulnar) side; the bone on this side is flattened 
by the pressure of the complete lateral digit, as may be seen in the sections of the four 
metacarpals given below the fig. 5. If Ave compare this part of the third meta- 
carpal with the corresponding region in Diplopus (fig. 6, hi.), Ave shall see a great 
difference. In this last genus, as there is no lateral second metacarpal, but only a rudi- 
ment of it, Ave find a deep depression, with small longitudinal facets, where the rudiment 
(seemingly a nodular bone, as in Anopl other ium) adhered to the third metacarpal. We 
meet Avith an exactly similar holloAV on the inner (radial) side of the third metacarpal in 
Xiphodon. On the upper and front part of this third metacarpal is a rough tuberosity for 
the tendon of the extensor carpi radialis muscle ; this is not so prominent in the same 
bone of Diplopus Aymardi (fig. 6), perhaps oAving to the younger age of the individual 
to Avhich the third metacarpal of fig. 6 belonged. 
The fourth metacarpal (Plate XXXVIII. fig. 5, iv.) of Hyopotamus , though repre- 
sented very badly by a proximal half of the bone belonging to a young individual 
from Puy, sIioavs, nevertheless, nearly all Ave require to knoAV about it. 
r \i\\e proximal surface is fiat, and has the shape of an isosceles triangle. On the radial 
upper margin Ave have a thickening at the point where the fourth digit is thrust under 
the ulnar prolongation of the third. The ulnar, or outer, side of the fourth metacarpal 
has two distinct facets, an anterior and a posterior one, for the articulation of the outer 
or fifth metacarpal. 
