OSTEOLOGY OF THE HYOPOTAMIDJ3. 
41 
The fibula of Diplopus (Plate XXXVI. fig. 7, and Plate XXXV. fig. 3) is shaped 
entirely in conformability with the Paridigitate type ; it is a considerably reduced bone, 
with a very thin shaft ; the distal extremity, however, is widened considerably in the 
antero-posterior direction. The inner surface of this broad (it would he more correct 
to say deep, as the fibula is widened from before backwards) distal extremity (Plate 
XXXV. fig. 3) is shaped on the same pattern as in the Suidse, presenting a more uniform 
arched and slightly raised platform pressed against the astragalus, and not a deep semi- 
circular notch as in Ruminantia. (In these the shaft of the fibula is generally mostly 
reduced to a mere tendon, but its lower extremity remains under the name of the so- 
called “ osselet peroneenJ') The articular end for the calcaneum (fig. 7') presents a very 
deep and narrow facet, which occupies the whole antero-posterior extent of the distal 
extremity of the fibula. The anterior part of this calcaneal surface is slightly convex, 
the posterior concave. 
I have found at Puy many distal extremities of the fibula, which belonged undoubtedly 
to Hyopotamus ; and even this bone bore a great resemblance to the same part of 
Diplopus. This is another fact testifying to the very close relationship of both genera, 
notwithstanding the difference in the number of toes. A resemblance which holds 
good even in such slight details is a conclusive proof that both genera — the Diplopus 
from Hordwell and the Hyopotamus from Puy (and Hempstead) — belonged to the same 
family, and that one may be considered the reduced form of the other, notwithstanding 
the seemingly adverse fact that the reduced form is met with in the Upper Eocene, and 
the more complete in the Lower Miocene. This last certainly had Eocene ancestors, 
which may be considered to have given rise to the reduced form, while they continued 
to live themselves until the Miocene period. 
Dimensions of Fibula, distal end. 
Diplopus 
(Plate XXXV. fig. 3, 
nat. size). 
Antero-posterior diameter of the upper broken end. . 
8 
Thickness of the same 
4 
Antero-posterior diameter of the distal end 
29, 26J 
Thickness of the same 
9 
General considerations on the carpal and tarsal , metacarpal and metatarsal hones in 
Hyopotamus, Diplopus, and other Paridigitata. 
If, generally speaking, the long bones of the limbs in Ungulata often present but 
few characters decisive enough to tell us at once the natural series to which an Ungulate 
mammal belongs, the converse is the case with the smaller bones of the extremities, which 
have therefore a great systematic importance. We may very often know most of the long 
bones of the skeleton, the scapula, the humerus, the antibrachium, the tibia and fibula 
of a fossil Ungulate, without being able to determine quite certainly the natural series 
to which it belonged ; nay, even more, we may discover the skull and the complete 
