OSTEOLOGY OF THE HYOPOTAMIDiE. 
43 
(Hyrax) to the largest Rhinoceros, Hippopotamus, or Giraffe, their osseous structure is 
exceedingly uniform and simple, presenting only two chief types, which are completely 
defined by the terms Paridigitata and Imparidigitata. Beginning with the oldest 
Eocene deposits, these two types are entirely separated by the structure of their skeleton, 
and we have not a single living or fossil form that could truly be considered a link 
between these two typically different series. I have little doubt that all Ungulates 
must have sprung from some common form ; but if so, their division was effected in 
very ancient times, perhaps in the early Cretaceous period. I say early , because- just at 
the close of the Cretaceous period, in the lignites of Soissons and the calcaire grossier, 
or in beds that are contemporaneous with them, we find large Mammalia of both 
series with a very reduced skeleton. Considering the rate at which the reduction of 
the limbs in Ungulates proceeded in the Eocene, Miocene, and Pliocene periods, we are 
obliged to grant a long time for the branching off of the two series of Ungulates from 
a common pentadactyle form, and the subsequent reduction of each branch to the three- 
and two-toed forms, met with in ancient Eocene beds. 
In the recent period, notwithstanding the great diversity and very wide distribution 
of Ungulates, they present an extreme poverty of type. The Imparidigitate series is 
strikingly poor, not only in generic but also in specific forms ; they go on evidently 
declining from the Eocene ; and in the recent period we have only three different types 
of these animals — the Equina, Bhinocerotina, and Taqyirina ; and even these three 
present such a fundamental resemblance in the structure of their limbs and skeleton, 
that no reasonable doubt can be entertained as to their descent from a common 
progenitor. On the other hand, the Paridigitata, though presenting in the recent 
period an exceedingly great diversity of size and habits and a great variety of specific 
forms, outnumbering tenfold those of the Imparidigitata, present also only two distinct 
types, Ruminantia and Suina ; and, even in these, the fundamental structure of the 
limbs and skeleton is so uniform that we may safely infer their descent from one 
common form. In the present memoir I have taken up only the second series, the 
Paridigitata ; and I shall try to show, as clearly as possible, the common bond that holds 
them all together. I shall point out that the uniformity of this fundamental type adapted 
to different conditions of life is so great that, notwithstanding extreme diversity of 
size, difference of habits, aquatic or terrestrial life, we may trace through the whole 
complexity of these diversified forms not only the number and shape of their carpal 
and tarsal bones, but even each separate facet of these bones, and point it out as clearly _ 
in the reduced limb of a land Antelope as it is displayed in the complete unreduced 
limb of the aquatic Hippopotamus. Further, I shall try to show that this uniformity 
of type holds good, not only of the living, but also of the extinct forms of Paridigitata. 
If we cast a glance on the first section of the fore foot, or the carpus, of any 
Paridigitate whose skeleton has not suffered too considerable a reduction (as for 
instance a Pig or Hippopotamus, Plate XXXVII. fig. 1), we shall find that it consists 
of two rows, containing seven small irregular bones, with an additional one (the 
MDCCCLSXIII. h 
