Prof. K. A. von Zittel—On the Mammalia. 405 
grade extremities, of which the terminal phalanges are provided 
neither with genuine hoofs nor with real claws, but with a some- 
thing intermediate between both; in all, the bones of the fore-arm 
and foot remain separate; the humerus is nearly always penetrated 
by an entepicondylar foramen; the femur has a third trochanter and a 
centrale was present in the carpus of most, probably of all the forms. 
The skulls, generally, have a depressed, longitudinally-extended 
form; strongly developed facial-bones, a diminutive brain-case, 
smooth cerebral hemispheres, which do not project over the 
cerebellum. There is, further, no very notable differentiation in the 
dentition. The incisors and canines are conical, the premolars are 
simple, and the brachyodont molars in the upper jaw tritubercular, 
and in the lower “ tritubercular-sectorial.” If it were possible to 
breathe life again into the animals of the Cernays and Puerco period, 
_ and to transplant them amongst the existing mammalian fauna, any 
zoologist would be very likely to group the Creodontia, Condylarthra, 
Pachylemuria, and Amblypoda of that period together into one single 
order, although they undoubtedly represent the primitive fore- 
runners of four, subsequently strongly-differentiated, groups. This 
growth of different stems from a common root, forms one of the 
strongest arguments in favour of the descent-theory, although it is 
also at the same time no small difficulty in the way of classification. 
If the Older Eocene mammals had not been further developed and 
differentiated, we should probably have distinguished only two 
orders of Placental mammals, of which one would be the Tillodontia, 
and the other would include all the remaining forms. 
Already in the succeeding zone of the Older Hocene, to which in 
Europe the London Clay of England, the Lower Sand, the Plastic 
Clay and Lignite of the Paris basin, belong, as well as also the 
so-called Wasatch or Coryphodon beds of Wyoming, Utah and New 
Mexico, in North America, the character of the Mammalian fauna 
has not inconsiderably been changed. The Allotheria have dis- 
appeared. The Creodontia have much increased in size, and are 
greatly differentiated, and they have already received a carnivorous 
stamp. Amongst the Ungulates, Amblypoda, Condylarthra, and the 
Perissodactyla are very distinctly separated; there are some rare 
and primitive forerunners of the Artiodactyla; the Prosimize 
(Pachylemuridz) are numerous, and the Rodents and Tillodonts 
are represented by typical and strongly-differentiated genera. The 
genera of this horizon, known up to the present, are distributed in 
Europe and North America, as follows: — 
ConDYLARTHRA. 
EUROPE. NorTH AMERICA, 
+Phenacodus.' Phenacodus. 
t Protogonia. Protogonia. 
+ Mentscodon. Meniscotherium. 
{Periptychus. LHyracops. 
1 The forms thus marked (f) are known only from the Bohnerz of Switzerland ; 
those printed in thick type are the specially characteristic and most numerously 
represented genera. 
