322 THE OSTEOLOGY OF ELOTHERIUM. 
and has referred the genus to the bunodont division of the family Anthracothervide, which 
family he derives*from an Eocene stock common to the Anthracotheriide, the Anoplothe 
rude, the Hippopotamide and the Suide (87, p. 80). 
The complete account of the dental and skeletal structure of Hlotherium is now 
before us and yet it is hardly less difficult than before to determine its phylogenetic 
relationships and systematic position. The genus is so far specialized that it implies a 
long ancestry, not a member of which is, as yet, certainly known, although there are 
certain Eocene genera which throw some light upon the problem. In the absence of this 
ancestral series, we are without any sure criterion by which to distinguish parallelisms 
from characters of actual affinity, since only by tracing, step by step, all the gradations 
of a differentiating phylum, can we safely determine the true position of its members. 
However, some facts seem to bear a clear and definite significance. In the first place, it 
is plain that Marsh is right in forming a separate family for this genus, as it belongs to a 
line which diverged very early from the main stem, whatever that was. In the second 
place, the relationship of this family to the Swidw must be a very remote one. When we 
compare the skeleton of Hlotheriwm with that of the swine and peccaries, point by point, 
the only notable resemblance between the two groups is found to consist in the bunodont 
character of the molar teeth, and this resemblance, standing by itself, cannot be regarded 
as at all decisive. The selenodont molar has been independently acquired by several 
distinct lines, and so far as the artiodactyls are concerned, the bunodont pattern is almost 
certainly the primitive one. That two widely separated families should each have 
retained a common primitive character is too frequent a phenomenon to excite surprise. 
In all other structures, skull, vertebral column, limbs and_ feet, no particularly close cor- 
respondences between the Hlotheriide and the Suidw can be detected, though that a 
common early Eocene progenitor should have given rise to both families is altogether 
likely. 
Between Llotherium and Hippopotamus, on the other hand, are many points of 
resemblance. The likeness in the dentition is here quite as great or even greater than 
between either of these genera and the Suide. In the skull there is much to suggest 
relationship, though combined with many striking differences, which may perhaps be 
referable to different habits of life, such as the enormous massiveness of the premaxillary 
and symphyseal region in the modern genus, the peculiar development of the canines and 
incisors and the elevated tubular orbits. In the skeleton the two genera are widely 
separated ; Hlotherium is a long-limbed, long-footed, didactyl creature, with small thorax 
and slender ribs, evidently of terrestrial habits. Hippopotamus, on the contrary, 1s a 
short-limbed, short-footed, tetradactyl and isodactyl form, with immense thorax and 
broad, almost slab-like ribs, which is chiefly aquatic in its habits. Whether the resem- 
