OSTEOLOGY OF THE HYOPOTAMIDA1. 
69 
far back as the Jurassic and even the Triassic period, have not yet furnished the slightest 
clue as to their skeleton), we meet always with Ungulates which belong unmistakably 
to the one or the other series ; a fossil Ungulate may be at once said to belong either 
to the Paridigitate or Imparidigitate division ; we have not a single form in which the 
characters of the two groups can be said to be mingled together, or which could be 
reputed to be the progenitor of both these great divisions of the Ungulata. Some of 
the early Eocene Paridigitata and Imparidigitata seem undoubtedly to be more nearly 
related, and have more common characters, than the greatly reduced forms we meet with 
in the recent epoch ; but this shows only, if we imagine these two series to be diverging 
lines meeting in some old Cretaceous Ungulate, that they occupy certain positions on 
these two different lines nearer to the point of divergence, but still so far from it that 
all the intermediate links had time to be completely destroyed before the Eocene 
period. We may imagine that in the Cretaceous epoch there existed an Ungulate 
form with a very complete skeleton and five digits to the manus and pes ; from this 
common form the divergence of Paridigitata and Imparidigitata may have been effected 
in such a way that, with the commencement of the reduction of the skeleton of this 
typical primary Ungulate, the chief development fell in one case on the middle (3rd) 
digit of the manus and pes, the laterals becoming more or less reduced and arranged 
symmetrically on both sides of this central digit of the manus and pes, so as to originate 
the series of Imparidigitata ; while in the other case the chief development fell on 
the two contiguous middle digits of the manus and pes (the 3rd and 4th), giving rise 
to the series of Paridigitata. But as the overdevelopment of a single middle digit, to 
such an extent as to support the body effectually, is a task much more difficult to the 
organism than the development of the two middle digits to a comparatively less extent, 
the reduction of the Paridigitate skeleton proceeded at a much quicker rate than that 
of the Imparidigitate; and while we meet with many animals of the Paridigitate 
series having only two digits in the Eocene, the Imparidigitata have always three. 
In the Miocene epoch, the two middle reduced digits of Paridigitata have already coal- 
esced to form a single digit, the cannonbone of Ruminants ; but the most reduced member 
of the Imparidigitata, the Anchitherium , still walked on three toes, though the laterals 
began to be greatly reduced ; and it is only in the later Miocene and the Pliocene periods 
that, with the appearance of the Eipparion and Horse, the skeleton of the Imparidigitata 
attained as great a reduction as the skeleton of Paridigitata did in the earliest Miocene, 
Gelocus (Aymard) being the first Paridigitate with a complete cannonbone in the adult. 
In the skeleton of this Ungulate progenitor, before the divergence of the two series was 
effected, we may imagine a pentadactyle manus and pes constructed in the way given 
in the following scheme. 
l 2 
