OSTEOLOGY OE THE HYOPOTAMID.E. 
37 
fig. 4, b). The two posterior small fossae for the radial facets of the ulna are not very- 
deep; they are separated by a posterior bony projection, which entered, or is thrust 
under, the connecting bridge of fig. 2, cb. We have no direct means of ascertaining the 
curvature of the radius, but it is given to us by the shape of the ulna (fig. 1), so that, at 
least in Diplopus , the radius must have been considerably arched forwards. 
The distal extremity (fig. o') of the radius is very unlike that of any existing Pari- 
digitate. In these we see generally on the distal end of the radius two longitudinal 
excavated facets, separated by an oblique prominent ridge running in the interspace 
between the scaphoid and the lunare (see Blainv. Ost. Anopl. pi. iii.) ; we have no trace 
of this ridge in Hyopotamus. The inner half of the distal end (fig. 3', i ) is occupied by 
the oblique convex facet for the scaphoideum ; the outer half presents a concave 
anteriorly broad facet for the lunare ; this bone, having a very oblique position in the 
carpus, encroaches by its posterior narrow prolongation upon the scaphoidal half of the 
distal extremity of the radius. Both facets are separated anteriorly by a deep groove, 
seen opposite the number 3' of the figure. The difference from the radii of all other 
Ungulata is considerable. To the external straight truncated surface (fig. 3', ex) the 
distal extremity of the ulna was articulated ; but I have had no specimen of Hyopotamus 
showing this distal end of the ulna. 
The radius of Hyopotamus was articulated (as in all Ungulates having a completely 
developed ulna) only with the two inner bones of the carpus, the scaphoid and the lunare, 
while the outer bone, the pyramidale, is taken up entirely by the distal extremity of 
the ulna. All existing Suit!® show us the same relation ; but in Dicotyles, whose skeleton 
is a little more reduced than that of the typical Pigs, we see that the distal extremity of 
the radius grows broader in consequence of the reduction of the ulna ; and, besides its 
two typical carpal bones, makes an encroachment upon the outer one, the pyramidale. 
With the still greater reduction of the ulna in most Ruminantia, the radius goes on 
increasing, and not merely touches the pyramidale, as in Dicotyles , but takes the whole 
half of its upper surface for its own support, while the reduced ulna is pushed back to 
the posterior half of the pyramidale. In consequence of this changed relation between 
the carpal bones and the radius, the distal extremity of this bone in Ruminants acquires 
at its outer border an additional facet for the pyramidale, of which not a trace is to be 
seen in Hyopotamus. We may mention as another peculiarity of the distal end of the 
radius of Hyopotamus , the entire absence of any styloid process or prolongation of the 
inner border of the distal end of the radius, which is well seen in other Paridigitates, as 
Anoplotherium and Hippopotamus. Specimens of the distal and proximal extremity of 
the radius which I possess from Puy agree in every particular with the one described 
from Hempstead. 
G 
o 
