517 
It has been long known that the earlier Tertiary strata furnish 
us with forms that partake of the character of groups which are 
now widely separated. M. Gaudry, however, in the volume I 
have before mentioned, gives us a great many illustrations of the 
way in which recent mammals, now distinct, are united by fossil 
forms. I believe that the importance and number of these facts is 
not so widely known as it ought to be, and I will therefore venture 
to instance a few examples, although the value and weight of the 
evidence can only be appreciated by those who will take tho 
trouble to examine it in detail. 
Ono of the principal divisions of the mammalia is that between 
placental and implacental animals. Two forms, of which only the 
jaws are known, Ptcrodon and Hytenodon , have been found in tho 
Eocene and lower Miocene of France. Some eminent naturalists, 
have ranked these animals, from their dentition, among the carnivora, 
while others equally qualified to judge, believe them, from certain 
characteristics which connect them with marsupials, to have been 
implacental animals, and this is exceedingly important, as connecting 
the secondary mammals, with their strongly marked implacental 
affinities, with those more highly developed forms which have, ex- 
cept in Australia and South America, now supplanted them. 
The connection which the study of extinct forms has established 
between pachyderms and ruminants, which were formerly regarded 
as quite distinct, has been such as to necessitate the reconstruction of 
the whole classification. M. Gaudry says, that the difficulty does 
not consist in knowing how ancient pachyderms became ruminants, 
so much as in deciding which species of the former have the best 
title to be considered the ancestors of the latter. Even the classi- 
fication of the mammalia which has been more recently adopted, 
that of the perissodactyla, or odd, and the artiodactyla, or even-toed 
animals, is not easy to insist on in the presence of fossil forms. 
The teeth of the ancient paridigitate Anoplotherium and the irapari- 
digitatc Pahvot/ieiium resemble each other so nearly, that it is 
difficult to distinguish isolated molars of these two animals. 
Among the proboscidians may be cited the case of a fossil 
pachyderm, which so partakes of the characters of the mastodon 
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