OSTEOLOGY OE THE HYOPOTAMI1EE. 
29 
only the dentition is known. Besides, we must not forget that the Hyopotamidce , as 
proved by the fauna of Mauremont and Egerkingen, are a true Eocene family ; and the 
presence of a large representative of this family in the Tapper Eocene of England is a 
very natural occurrence. I stated before that, even in the Mauremont and Egerkingen 
fauna, some of the Hyopotamidce seem to have been didactyle ; and we may take this 
as pretty certain as regards the small Hyopotamus Uenevieri. The new didactyle animal 
from Hordwell is only a larger representative of these Eocene didactyle Hyopotamidce ; 
and we may expect that when its dentition is completely known it will very probably 
resemble that of the Eocene Hyopotarni as found at Mauremont and at Egerkingen, 
.and differ by its more ruminant-like premolars from the true llyopotami which occur 
at Hempstead and in Puy. The presence of didactyle genera in Eocene deposits, 
while we find tetradactyle genera belonging to the same family in the Miocene, is no 
objection to the theory of descent as it is often argued by the adherents of the 
special creation hypothesis. The primary stock was undoubtedly tetra- or even penta- 
dactyle ; and under the incessant tendency to greater reduction and simplification of the 
limbs, which we witness in all Ungulata without exception, there were given off side 
branches which reached this utmost reduction of the limbs in the Eocene and became 
extinct, while the original unreduced stock continued to live till the Miocene period. 
A similar case may be imagined in relation to the recent Suinee. There is no doubt 
that the Dicotylidce represent one of the most advanced and reduced branches of the 
family of Suina ; they practically reach nearly the same state of reduction of their limbs 
as the didactyle genera, their lateral digits being only useless appendages, having no 
importance for locomotion. Imagine that, by some geological change, the Dicotyles 
should become extinct in South America, while other continents should continue to be 
peopled by unreduced typical Suina : in this supposed case we should have an extinction 
of the filial branch, while the parent stock would continue to live and flourish. In the 
same manner I have little doubt that the didactyle Hyopotamidce found at Hordwell 
and Mauremont descended from a tetradactyle stock, which very probably presented the 
same structure of the skeleton as we find it in the Miocene Hyopotamidce from Puy and 
Hempstead*. And though the didactyle genus is found in strata older than those 
which gave us such complete materials for the restoration of the tetradactyle Hyopota- 
midce , still, seeing the similarity of their skeleton, we may consider the didactyle genus 
from Hordwell as a reduced descendant of a form very similar in its skeleton to the 
llyopotami of Puy and ITempstead. 
Having ascertained the existence of this reduced representative of the family of the 
Hyopotamidce , I could not, in view of the difference in the number of digits, permit 
the new form to remain in the genus Hyopotamus ; and though I strongly object to the 
creation of new names, there is no help for it in this case, and a new generic division 
must be made to receive the didactyle Hyopotamoid from Hordwell, as well as similar 
forms which may turn up in the future. As the chief distinction of this genus is its 
* The probability is converted nearly into complete certainty if we consider that the reduced didactyle genus 
presents rudiments of two additional digits, the second and fifth. 
