152 AMERICAN MESOZOIC MAMMALIA 



MIDDLE Stereo gnathus 



JURASSIC 



Am-philestes 



P hascolotherium 



Antfhithermm 



RH AET I C Tritylodon Tritylodon 



Oligokyphus 

 C halefotherium 



Archaeodon 



Microcleftes 

 Tkomasia 



Hy-psi-prymnofsis 



The Morrison of North America and the Purbeck of England are almost or quite 

 contemporaneous. The Amphidontidae are as yet unrepresented in the Purbeck, but 

 members of this family are excessively rare in the Morrison and their apparent absence 

 in England is no doubt due to lack of discovery. Aside from this, all the known fami- 

 lies are common to the two faunas and are in about the same stage of evolution in each. 

 Ctenacodon among the multituberculates, Trioracodon among the triconodonts, and 

 Amblothermm among the pantotheres occur in both, and the other genera, while quite 

 distinct, are for the most part closely comparable. The correspondence between the two 

 faunas is sufficiently close to warrant the following three conclusions : 



1 . The Purbeck and the Morrison mammalian faunas are of similar facies. 



2. They are of nearly or quite the same age. 



3. They were apparently derived from a common center and indicate that an 

 avenue of migration practicable for small mammals linked England and western 

 North America some time in the Upper Jurassic, either directly or through this com- 

 mon center. 



The differences are all explicable on the basis of wide geographic separation, in 

 view of the presumably feeble migratory powers of the mammals, without postulating 

 any definite intervening barrier or any marked difference in facies. 



SUCCESSION OF EARL Y MAMMALIAN 



FAUNAS 



The known distribution in time and succession of the Mesozoic orders and fami- 

 lies is shown in the following table : 



