322 THE OSTEOLOGY OF ELOTHERIUM. 



and has referred the genus to the hunodont division of the family Anthraeotheriidce, whieh 

 family he derives from an Eocene stock common to the Anthraeotheriidce, the Anoplothe 

 riidce, the Hippopotamidce and the Suidce ('87, p. 80). 



The complete account of the dental and skeletal structure of Elotherium is now 

 before us and yet it is hardly less difficult than before to determine its jffiylogenetic 

 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 Suidce must be a very remote one. When we 

 compare the skeleton of Elotherium 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 Elotheriidce and the Suidce can be detected, though that a 

 common early Eocene progenitor should have given rise to both families is altogether 

 likely. 



Between Elotherium 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 Suidce. In the skull there is much to suggest 

 relationship, though combined with many striking differences, whieh 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 ; Elotherium is a long-limbed, long-footed, didactyl creature, with small thorax 

 and slender ribs, evidently of terrestrial habits. Hippopotamus, on the contrary, is a 

 short-limbed, short-footed, tetradactvl and isodactyl form, with immense thorax and 

 broad, aim st slab-like ribs, which is chiefly aquatic in its habits. Whether the resem- 



