ON THE EXTINCT ANIMALS OF NORTH AMERICA. 
293 
In the character of the molar teeth, of which there were a 
considerable number resembling one another in form, these 
animals, and many others less perfectly known, resemble the 
well-known Hycenodon of Europe, a lost type of carnivorous 
animal first found in the Upper Eocenes of Europe, but abund- 
ant also in America at an apparently later age. The members 
of this group of carnivores are all characterised by long and 
somewhat slender jaws, containing a series of teeth one behind 
the other, each being in its form a repetition of the one before 
it, as in many of the existing predaceous marsupial animals. 
The greater differentiation of the characters of the teeth and 
the shortening of the jaws, with corresponding increase of the 
force with which they can be closed, seen in the highest forms 
of modern carnivores, is one among many examples of pro- 
gressive adaptations conducing to more complete efficiency in 
performing the functions of life. These Eocene carnivores also 
(according to Cope) show a primitive character in the tibio- 
astragalar articulation, or “ ankle-joint.” “ The astragalus is 
flat, and the applied surfaces are nearly a plane, and without 
the pulley-shaped character seen in existing carnivora, as dogs, 
cats, and, in a less degree, in the bears and in other mammalia 
with specialised extremities, as Perissodactyla , Artiodactyla , 
&c. The simplicity of structure resembles, on the other hand, 
that found in the opossum and various Insectivora , Roclentia , 
and Quadrumana , and in the Proboscidian most of which 
have the generalized type of feet. The structure indicates that 
the carnivorous genera named were plantigrade — a conclusion 
which is in conformity with the belief already expressed, that 
the mammalia of the Eocene exhibit much less marked ordinal 
distinction than do those of the Miocene or the recent periods. 
It is, indeed, questionable whether some of the genera here in- 
cluded in the carnivora are not gigantic Insectivora , since the 
tibio-tarsal articulation in many, the separation of the scaphoid 
and lunar bones in Synoplotherium, the form of the molars, 
and the absence of incisor teeth in some, are all characteristic 
of the latter rather than the former order.” 
The Miocene carnivorous animals found associated with the 
herbivorous Oreodons of Dakota are more perfectly known, 
many of them having been well worked out and figured some 
| years ago by Leidy. The most remarkable are several species 
of Hyaznodon , a genus already mentioned as found in the 
Upper Eocenes and Lower Miocenes of France, and also of the 
south of England ; but one of the American species {H. horri- 
dus , Leidy) is larger than any of its European congeners, its 
skull (which, as Leidy remarks, is not like that of any existing 
carnivores, but something intermediate between that of a wolf 
and an opossum) fully equalling that of the largest individual 
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