OSTEOLOGY OF THE HYOPOTAMIDiE. 
49 
also in Phacochcerus and Babirussa ; it seems to hang only on the back part of the 
trapezoid (t), without touching the second metacarpal. The trapezium is very small 
in Dicotyles (Plate XXXVII. fig. 5, tz), and is entirely lost in ruminants, not being even 
present as a distinct point of ossification in the trapezoido-magnum cartilage. It is 
worthy of notice that, though the metatarsus in all Ungulata is always more reduced 
than the metacarpus, still the homologue of the trapezium (the first cuneiform) is often 
found in the pes when no trace of its homologue exists in the manus. So, for instance, 
in the Horse (coalesced with the second cuneiform) and most, if not all, Ruminants, in 
which the first cuneiform is present, while the trapezium is lost. 
It is difficult to say if it existed in the Diplopus , seeing the reduction of its meta- 
carpals to two ; but as it exists in Anoplotherium there is no reason why it should be 
lost in the two-toed Diplopus. 
The trapezoid (Plate XXXVIII. fig. 5, t). — I have this bone from Puy, belonging to 
Hyopotamus or the four-toed form. In considering only the shape of this trapezoid, 
without heeding the size, it is almost identical with the trapezoid of Hippopotamus ; 
only its upper or proximal surface is more convex. The distal surface is deeper than it 
is broad, slightly concave, and fits exactly the proximal surface of the second metacarpal 
(Plate XXXVIII. fig. 5, t). On its radial side is seen a well-developed articular facet 
for the trapezium. If we compare this trapezoid with that of a pig (Plate XXXVII. 
fig. 4, t), we shall find a great difference, especially in the distal surface. In Hyopotamus , 
as well as in Hippopotamus , the trapezoid is destined to support only its typical 
metacarpal (the second), while in the Suidae, owing to the larger development of the 
middle, and consequent reduction of the lateral metacarpals, one half of the distal 
surface of the trapezoid is taken by the third metacarpal, and only the remaining 
half supports the second (Plate XXXVII. fig. 4, t). In consequence of this, the 
distal surface of this bone in the true Suidae, instead of being flat, is spear-shaped, with 
a sharp edge running through its whole antero-posterior depth. In Dicotyles (Plate 
XXXVII. fig. 5, t), where the reduction of the lateral metacarpals has gone still further, 
the distal surface of the trapezoid, being wholly taken by the third metacarpal, has 
resumed its flat form, thus resembling more the trapezoid the Hyopotamus than that of 
the pig. But notwithstanding this similarity, the part played by this bone in Dicotyles 
and Hyopotamus is wholly different ; and while in the first it has no connexion with the 
second metacarpal, in the last it is entirely connected with it, and gives no facet to the 
third metacarpal. 
Dimensions. 
Height ........ 101- 
Breadth, inf. 8 
Antero-posterior depth . . . 15 
Unfortunately the os maynum is wanting in all the collections I have visited; but 
the interval included between the surrounding bones gives an idea of its shape, which 
