H. F. Osborn — Mammalia in North America. 453 



Breaks and Links in the Mesozoie Fauna. 



By our hypothesis all three sub-classes flourished together 

 during the American Mesozoie ; the Marsupials disappeared, 

 then the Monotremes, and by the end of the basal Eocene the 

 Placentals were in exclusive possession of the northern con- 

 tinent. 



Jurassic Insectivores 



Lemurs 



Monkeys 



Modern Carnivores 



I C P M 



Examples of Dental Curves. 



Although we have great reason to congratulate ourselves 

 upon the rapid progress of discovery, there still remain great 

 gaps in Mesozoie time between certain horizons and in the 

 lineal phyletic series of both the Mesozoie and Cenozoic. For 

 a time standard we may take advantage of the remarkably 

 constant evolution of the Plagiaulacidse in the Mesozoie, and 

 of the Equidse in the Cenozoic — as certain invertebrates are 

 made use of in older rocks. The grooves and tubercles of 

 Plagiaulax and the cusps and styles of the horses are added 

 with the precision of clockw r ork, and supposing that the rate of 

 evolution has been about the same, we can approximately esti- 

 mate both the periods of deposition and the intervals as below. 



Plagiaulacidae. 









Stonesfield. Purbeck. 

 ? 4-3 

 ? 7-9 



; ? 4:2 



Laramie. 



2 



11-14 



6: 4 



Puerco. 

 2-1 



12-15 

 6: 4 



Cernaysian 



1 



14 



9:6 



Number of Premolars, 

 Grooves on Premolars, 

 Molar Tubercles : outer ; inner 



Estimating the geological intervals by dental evolution and 

 f aunal succession, there is first the great gap between the Trias 

 of Microlestes and Dromatherium and the Jurassic of the 



