INTERPRETATION OF THE VERTEBRATE FAUNAS 391 



1. Lapse of time. 



2. Difference of local environment. 



3. Migration movements representing a change in environment some- 

 where else, not necessarily in the region concerned. 



Lapse of time will be represented by changes in the evolutionary stages 

 of the phyla which pass through from one to the other. It will not bring 

 about sudden changes in the composition of the fauna, although certain 

 phyla may diminish, while others increase in numbers. 



A difference of local environment will involve the absence of certain 

 groups of more or less restricted habitat, whose continued existence may 

 yet be known by their presence in both earlier and later faunae of differ- 

 ent facies, or may be inferred from more indirect evidence. 



A migration movement may cause the sudden appearance of new 

 groups, the disappearance or extinction of older ones, with or without 

 any apparent change in the facies or environmental type of the faunae 

 compared. 



In attempting to apply the vertebrate evidence to correlation of the 

 later Cretaceous and earlier Tertiary formations, these principles must be 

 kept in mind or the results will be misleading. If properly understood 

 they serve to reconcile what have appeared to some authors to be contra- 

 dictory statements by vertebrate paleontologists. Marsh in his descrip- 

 tion of the Lance fauna lays weight on its resemblance to the Jurassic 

 and Lower Comanchic faunae. This is quite correct, inasmuch as it con- 

 sists of Multituberculates unknown in the Tertiary except for a few 

 Paleocene survivors, and of Trituberculates of rather remote affinity to 

 the Tertiary placentals and similar in several features to those of the 

 Jurassic. , Osborn, on the other hand, pointed out that the Multituber- 

 culates of the Lance were much more closely allied to their PaJeocene 

 successors than to their Jurassic ancestors, and rightly concluded that 

 there was no wide time-gap between the Lance and the Paleocene. The 

 evidence as presented by Marsh and Osborn is not conflicting, but it pre- 

 sents different aspects of relationship. The Lance mammal fauna is' 

 near to that of the Jurassic in facies ; it is much nearer in time to the 

 Paleocene faunae. 



Comparing the Belly River and Lance faunae, we find evidence of a 

 considerable gap in time as represented especially by the progressive 

 evolution in the Ceratopsia and other dinosaurs. But there is no good 

 evidence of any change in facies or of the appearance of new immigrant 

 groups of reptilia or mammalia. The reptilian phyla one and all con- 

 tinue through, some with little change, others with more considerable 

 progressive evolution. The scanty evidences of mammals from the Belly 



