OSTEOLOGY OF THE HYOPOTAMIDAE. 
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On the posterior edge of this second metacarpal is a small facet, probably for the 
trapezium ; although I did not find this last bone, its existence must be inferred from a 
facet on the trapezium and this posterior metacarpal facet. 
The general shape of the bone is somewhat triangular ; on its flat side it is pressed 
against the third metacarpal. 
The fifth metacarpal . — Of this bone I have seen only somewhat less than the upper 
third. As shown by this small fragment, the fifth digit was relatively well developed ; 
our fragment probably comes from a larger individual than the other metacarpals. 
The proximal surface has a facet which occupies its entire antero-posterior depth and 
articulated with the unciform (fig. 5, v.); the inner side shows two separate facets, one 
anterior and one posterior, which entirely correspond with similar facets on the outer 
side of the fourth metacarpal (the Hog and Hippopotamus has only one such facet). The 
outer edge of this fragment is thickened and rugose for the attachment of ligaments. 
The Metacarpus of Diplopus. 
The chief differences exhibited by the two middle metacarpals of the didactyle 
Diplopus (Plate XXXVIII. fig. 6) have been to a certain extent already stated in the 
course of the discussion of the metacarpals of Hyopotamus ; something may, however, 
be added. The general shape is exceedingly different, as may be seen by the sections. 
The two metacarpals of the didactyle Diplopus are much more mutually symmetrical 
than those of Hyopotamus ; the contiguous sides are flattened in such a way that the 
two bones are pressed together on their flat surfaces, while their outer rounded outlines 
sweep in and out in a nearly regular quadrant, so that the two united metacarpals 
represent in section the half of a solid cylinder. These inner flattened faces are very 
rough, showing the attachment of numerous ligaments that held them firmly together. 
The distal extremity is turned a little outwards and broadened transversely ; its inner 
half is much thicker or deeper than the outer, more so than in the recent Suina. 
The distal articular ridge of the metacarpals is limited only to the posterior or palmar 
side, although a faint trace of it is visible even on the anterior face of the distal 
extremity of the metacarpals. The semicircular line, where the anterior surface of the 
metacarpal passes into the distal articulation for the first phalanx, is only slightly 
excavated, while in the metatarsals this line presents a deep crescentic concavity ; this 
furnishes a very good practical character for distinguishing broken distal ends of the 
metacarpals from the metatarsals. 
We have already mentioned the difference in the proximal surface of the third 
metacarpal, by the radial edge (fig. 6, hi.) being more raised, by the more horizontal 
direction of the ulnar process, and by the depth of the excavation, into which is 
fitted the radial projection of the fourth digit. The posterior surface of the meta- 
carpal is flat, as seen in the section. The fourth metacarpal (Plate XXXVIII. fig. 6, 
IV.) shows similar differences ; the proximal surface is of a rounded triangular outline 
to fit the similar facet of the unciform (fig. 8 f , iv.) ; the projection on the radial flat 
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