OSTEOLOGY OF THE HYOPOTAMIM. 
65 
the Museums of Lausanne and Puy I had no doubt whatever that the genus imperfectly 
known under the names of Hyopotamus and Botliriodon had four completely developed 
digits on both extremities ; and all I had seen in the collections of Puy confirmed this 
inference; besides, as I could not find any difference in the dental or osteological 
characters between these two supposed genera, I was obliged to unite them, giving 
priority to Professor Owen’s denomination. But, whilst studying the fossil remains 
from Hordwell and Hempstead in the collection of the British Museum, I gradually 
acquired the conviction that we have, in the English deposits, two very different forms of 
the same natural family, which, though entirely similar in most of their osteological 
characters presented a wide difference in the composition of their manus and pes, 
a difference somewhat similar to the difference we see in our own times between 
Hyomoschus and the true Kuminants. One form had clearly four completely developed 
digits, while the other had only two. At first I struggled against this conclusion, as the 
fact seemed too singular, considering the similarity of all the other bones of the skeleton ; 
but by-and-by, as I got more materials, all doubt cleared away ; and having once ascer- 
tained this point, I felt obliged to subdivide the family of Hyopotamidse into two 
groups or genera, Biplopus and Hyopotamus, whose chief distinction lay in the number 
of digits. It is more than probable that each group or genus was represented by 
several species, though this specific distinction is very difficult to establish. But I shall 
return to this question at the end of my memoir ; at present I proceed with the descrip- 
tion of the bones forming the metacarpus and metatarsus in both genera. As I have 
generally done, I commence with the tetradactyle form, or the Hyopotamus , and shall 
proceed afterwards to the didactyle, or Biplopus. 
Before, however, proceeding with the Hyopotamidse, it seems necessary to cast a 
glance at the composition of the manus and pes in all Paridigitata, living and fossil, 
as such a general review will throw more light on the importance and value of the 
distinctions found in our fossil Hyopotamidse than a tedious and minute description 
of their bones could do. Studying in detail the structure of the extremities of the 
living and extinct Ungulata Paridigitata, and especially their metacarpals and meta- 
tarsals, we meet everywhere, through all the great diversity of their forms, such striking 
similarity in all the smallest details, such adaptation of one typical structure to widely 
different conditions, that there seems to be no possibility of explaining it otherwise 
than by a common descent (with modification and adaptation) from some single ancient 
form which presented the arrangement we call typical for the whole of the Paridigitata. 
Looking upon the whole community of Paridigitata as modified descendants of one 
ancestral form, the theory of evolution says that each form must have inherited the 
typical structure of the common ancestor, modifying it in each particular case to the 
condition of its own life ; and though, after the lapse of a great period of time, the 
diversity acquired by different forms would be prodigious, still every member of the cycle, 
even at the furthest points of descending lines radiating from the common ancestor, would 
by every single bone testify its relation to this common ancestor. And such is really the 
