ORIGIN AND DISTRIBUTION OF MAMMALS 295 



Oxyaenidae, MesomjcJiidae, Paraviys of the rodents, Hyrachyus, Amynodon, 

 Heptodon, Dolichorhinus and Telniatherium of the perissodactyls, Achaenodon of 

 the artiodactyls and Leptotragulus of the earliest Camelidae. Particularly 

 characteristic of this fauna is the presence of the last dinoceratid, Eohasileus. 

 In the somewhat more recent Uinta fauna we find the last Miacidae, 

 Oxyaenidae, Mesomjchidae, the first Canidae, Procynodidis, and the last of the 

 Hyopsodontidae. A new rodent type, possibly one of the Geomyidae — Protoptychus 

 — appears to take the place of Paramys, a geiuis which is gradually disappearing. 

 The perissodactyls have a somewhat greater variety of forms than in the 

 foregoing fauna, for with the forms of Amynodontinae and Palaeosyopinae — 

 Diplacodon, Telniatherium, Manteoceras — which appeared at an earlier time, are 

 associated a tapir, Isedoloplms, and the Epihippus, the genus of Equidae which 

 is so extraordinarily important in the developmental history of the horse. 

 The hyracodontid — Ryrachyus — however is substituted by the more slimly 

 built form, Triplopus. The artiodactyls also undergo a considerable increase 

 in forms. Instead of the extinct genus Achaenodon the first Agriochoeridae — 

 Protoreodon and Protayriochoerus — appear and the Camelidae become more 

 numerous — Leptotragidus, Protylopus, Camelomeryx. The genus Bnnomeryx, 

 which probably is the forerunner of Protoceras, also appears. 



The careful study of these faunas of the northern hemisphere has in no 

 way confirmed the supposition that a considerable exchange of forms took 

 place between North America and Europe during the whole Eocene, for the 

 number of forms common to both continents is increasingly small. But these 

 researches have shown that there are a large number of co-ordinate forms 

 which follow the same or a very similar progressive development on both 

 continents, and that the presentation of a number of unusually complete 

 genetic sequences is possible. As a rule, however, no gradually increased 

 branching from a single type form is found, as might be supposed, but many 

 species in one genus arise simultaneously, and these then form parallel lines 

 of development. The careful researches upon the European perissodactyls 

 and artiodactyls, for which we are indebted to Stehlin, present many such 

 genetic lines of development, which extend sometimes from the Lower Eocene 

 up to the Oligocene. The generic lines of development are indicated here 

 only in the genera Choeropoianius, Cebochoerus, Choerom,orus, Ehagatherium, 

 Diplobune, Pseudamphimeryx, Dichodon, Anchilophus, Palaeotherium, Paloplotheriujn 

 and Lophiodon. The study of the creodonts, the rodents and the primates 

 will aid us soon to complete this series. Osborn, Wortman and Matthew 

 have succeeded in completing similar lines of development from material 

 found in North America. These studies have been made to include not only 

 the perissodactyls and artiodactyls but also the primates, the creodonts, 

 the rodents, the Ganodonta and the Amblypoda. In contemplating these 

 parallel lines of development it is particularly surprising to find in Europe 

 at least the origin of a new genus from geologically older genera exceptional. 

 This can scarcely be explained otherwise than by repeated immigration from 

 a centre of development as yet unknown but which is sought at times in Asia, 

 in an Arctic continent, and in the north-eastern part of North America. This 

 is a problem which can only be solved by future discoveries. The advances 

 made along these parallel lines of development are apparent above all in the 

 great increase in bulk, among the perissodactyls and sometimes among the artio- 

 dactyls, in the complicated structure of the premolars, in the specialisation of 



