OSTEOLOGY OF THE H Y OPOTAMIDiE. 
35 
very large and broad olecranon, which differs slightly from living Ruminants and Pigs by 
its greater transverse breadth, and by the more projecting hook of the fore part, which 
entered deeply into the intercondyloid perforation (fig. 4, c) of the humerus. The fossa 
sigmoidea is uninterruptedly united on the inner side to the radial facet, while its outer 
part presents a deep ecliancrure. The outer radial facet of the ulna is not united to 
the fossa sigmoidea in the Dijilopus, though it is so in the smaller Hyojpotamus from 
the Isle of Wight and from Puy (fig. 2, or). Both articular facets for the radius are 
nearly in one plane, whilst in Ruminants the outer facet projects much more forwards 
than the inner. The section of the ulna of Diplojpus gives us a figure like a fiat or low 
triangle, not quite regular on its external border. The posterior surface (Plate XXXVI. 
fig. l",y>) of the ulna is the large basis of the triangle ; the obtuse apex (a) presents a 
rugose ridge, seen in fig. 1', running on the fore part of the ulna, and by which it is pressed 
against the radius ; the sides of the ulna are inclined planes, uniting the extremities of 
the base to the apex. Just below the second third of its length the three planes of the 
triangular ulna begin to narrow whilst descending to the distal extremity, which retains 
the same triangular outline. The inferior extremity is cut nearly at right angles by the 
facet for the outer bone of the carpus. 
If we compare smaller ulnae from Puy and the Isle of Wight we shall see some dif- 
ferences not only in the shape of the sigmoid fossa, but likewise in the transverse section 
of the ulna. In some of the ulnae, like the one figured from Puy (fig. 2), the lower and 
fore part of the fossa sigmoidea is much produced forwards, forming a sort of prominent 
bridge between the inner and outer radial facets of the ulna ; the production of this 
connecting bridge creates a deep fossa in the middle of the anterior surface of the ulna. 
Having examined this part of the ulna in a large number of recent Suina, I found it 
very variable, and therefore cannot lay too great a stress upon it in the Hyopotamidce . 
But besides this, the ulnae from Hempstead and Puy, belonging undoubtedly to Hyo- 
jpotamus , have a very different horizontal section, and they are by no means so much 
flattened as the ulna of the Diplopus ; this difference is most clearly seen by comparing 
the horizontal sections of both ulnae taken about the middle of the bone. The species 
from Puy have a much deeper and sharper posterior edge (fig. 2', ^), which is nearly 
absent in the Diplopus. Unfortunately I have not found a single entire ulna of the 
Hyopotamus , though I have seen a large number of broken specimens from Pay and the 
Isle of Wight. Comparing these broken ulnae with the one figured, I found some 
notable differences in details, which will be clear to the reader by comparing the figure 
of a part of the ulna from Puy and its transverse section (figs. 2 & 2') with that of the 
ulna from Hordwell (figs. 1 & 1'). The differences extended further than the shape of 
the sigmoid cavity, as the following Table of measurements will show. 
MDCCCLXXIII. 
G 
