THE EOCENE OF EUROPE AND NORTH AMERICA 109 



Thus the archaic forms predominate in the ratio of forty-one species of 

 archaic to one species of modern affinities. This rich fauna is beheved to 

 be of the same age as that of a portion of the Fort Union of Montana, as 

 descril)ed by Doughiss ' (1902) and Farr. Its approximate parallels in 

 Europe (Upper Thanetian or Cernaysian) are indicated by the common 

 presence in France and North America of somewhat similar stages of evolu- 

 tion among the representatives of three or four families, namely: (1) Pla- 

 giaulacidae, (2) ArctocyonicUe, (3) Mesonychidie-Triisodontinge, (4) Oxy- 

 clainidae. As noted above, other identifications of the Torrejon and 

 Cernaysian faunas are somewhat uncertain. 



As in the Puerco, these Torrejon mammals belong almost exclusively to 

 an older radiation, destined to become extinct during the Eocene. This 

 elimination, in fact, begins at once, because five out of the fourteen families 

 of mammals discovered in the Torrejon make their last appearance at this 

 stage. The remark applied by Dr. Lemoine to the Cernaysian fauna, 

 "Comme c'est drole, ce monde Id,^' certainly applies with equal force to the 

 Torrejon world; it was certainly strange and bizarre, none the less ex- 

 tremely interesting and fortunately much more completely known than 

 the Puerco assemblage, because the limbs and feet of several of its mem- 

 bers have been discovered. 



It was the happy finding of the fore foot of Psittacotherium which led 

 Wortman " to the demonstration that this member of the family Stylinodon- 

 tidse, as well as the animal known as Conoryctes of the order Taeniodonta 

 (Ganodonta), are strongly analogous if not actually related to the South 

 American gravigrade Edentata, such as Megalonyx, and the armadillos, 

 respectively. 



These browsing or leaf-eating taeniodonts now attain a considerable size, 

 and present a direct passage between the Wortmania ("Hemiganus'^) of 

 the Puerco and the Calamodon of the Wasatch or Sparnacian. There are 

 still no true rodents. Beside the Mioclaenidae (Mioclcenus) and Panto- 

 lestidae {Pentacodon) there are the Mixodectidae, making their first 

 appearance with a pair of greatly enlarged incisor teeth, which suggested 

 to Cope their affinity with the Chiromys, or Aye- Aye, of Madagascar, and 

 to Osborn their possible relationship to the Rodentia (Order Proglires). 

 These small insectivore, rodent, or lemur-like forms are destined to survive 

 to the summit of the Eocene. 



It is noteworthy how frequently diprotodonty, or the enlargement of a front 

 pair of incisor teeth, appears not only in the marsupial suborder ' Diproto- 

 dontia,' but as a parallel or analogous adaptation in these Basal Eocene mam- 

 mals of Europe and America, and in the several families of other orders. 



' Douglass, E., A Cretaceous and Lower Tertiary Section in South-Central Montana. 

 Proc. Amcr. Philos. Soc, Vol. LXI. 1902, pp. 207-224. 



^ Wortman, J. L., The Ganodonta and their Relationship to the Edentata. Bull. Amer. 

 Mus. Nat. Hist., Vol. IX, 1897, pp. 59-110. 



