292 ORIGIN AND DISTRIBUTION OF MAMMALS 



The difficulties connected with a systematic discussion of this period can 

 be scarcely overcome, for the structure of many of these forms is very primitive, 

 the dentition is almost always composed of incisors, canines and premolars 

 which are very similar to those of carnivores, the molars are bunodont or 

 secodont — -the lower ones more or less tuberculo-sectorial and the upper ones 

 tritubercular, — the bones of manus and pes are almost alwaj's of very in- 

 definite shape, the humerus has an entepicondylar foramen and the femur 

 often has a third trochanter, and the powerful fibula does not meet the 

 calcaneum, Manus and pes have five digits and are, as far as known, 

 somewhat plantigrade. The end-phalanges are developed neither as a real 

 hoof nor as a real claw. It is therefore hardly to be wondered at that so many 

 species have several times changed their position not only within the genus 

 but also wuthin the order, and that even the divisions between families and 

 orders have often been shifted. 



These facts furnish a weighty argument for the truth of the theory of 

 evolution, since the uniform structure of the earliest placentals permits no other 

 conclusion than that all or at least most of the orders of higher mammals 

 may be traced back to one or the other of very few primitive forms. The 

 fauna of Rheims and of the Puerco and Torrejon beds is usually designated 

 at the present time as Basal Eocene. 



The Lower Eocene comprises in Europe the Sparnacian and Ypresian 

 London Clay, the Teredina sands of Epernay, the Landenian of Erque- 

 linnes and Orsmael in Belgium, and the Plastic Clay and Lignite of the 

 Paris Basin ; in North America the Wasatch beds of Wyoming and New 

 Mexico and the somewhat more recent Wind River beds of Wyoming and 

 Colorado. In Europe the affiliations with the older fauna of the Cernaysian 

 still seem to be close, at any rate the Ageian and Landenian still contain 

 Plesiadapis, Protoadapis, Dissacus and Didymidis. Notwithstanding, true 

 rodents — Plesiardomys, — well-developed Creodontia — Protoprnviverra, Palaeonidis, 

 Pachyaena, — true perissodactyls — Hyracotherium, Propachynolophiis, Pachynolophus, 

 Chasmotherium and Lophiodon, — an artiodactyl — Protodichobune, — and according 

 to some also a condylarthran — Phenacodus ? — as well as an amblypod genus 

 CorypJiodon, which reaches the zenith of its development in the Wasatch and 

 Wind River beds, all already appeared at this time. Whereas in Europe only 

 the artiodactyls and perissodactyls, the rodents and certain creodonts present 

 a new link in the history of mammals, in North America the gap between 

 the fauna of the Basal and Lower Eocene seems of somewhat greater significance. 

 Real primates — Anaptomorphidae, Pelycodus, Nothardtis,- — numerous insectivores 

 Avhich are very closely related to the primates — Mixodedidae, — and the 

 Hyopsodontidae, rich in genera and partly deviating from the true insectivores 

 on account of size — Esthonyx, PalaeosinojM, — partly very similar to them — 

 Palaeidops, — also the earlier rodents — Paramys, — and almost all the families of 

 the later Creodontia — Palaeonididae, Oxyaenidae, Hyaenodontidae and Mesonychidae ; 

 all these forms appear in the North American fauna of this period. The 

 Miacidae also develop a considerable variety of forms. Those that appear for 

 the first time are the Meniscotheriidae, also many perissodactyls — Heptodon, 

 Systemodon, Eohippus and Lambdotheriuvi, — and certain artiodactyls — Trigo- 

 nolestes, Parahyv^. On the other hand the Ganodonta — Calamodon, Sfylinodon, — 

 Coryphodon of the amblypods, and the Phenacodontidae of the Condylarthra are 

 all quite closely allied to forms of the Torrejon, The ditference between the 



