598 HENRY SHALER WILLIAMS 



to shift their locus habitans. One fauna (the Ithaca) has either 

 become partially destroyed or forced to migrate, while the other 

 (Chemung) has entered for' the first time into the region where the 

 New York state rocks were being deposited. 



In some particular section the change of occupation may take 

 place suddenly so that the last of the Ithaca species will be brought 

 into close contact with the first appearance of the Chemung, but 

 this state of facts cannot be expected without some violent revo- 

 lution, of which no evidence is here given. In some cases, particu- 

 larly east of Ithaca, it is difficult to distinguish the passage, but for 

 the reason that there the Ithaca fauna is in a later state of its evo- 

 lution than at Ithaca, and thus the transition of species between 

 the two homeotopic faunas is less conspicuous. This is due to the 

 fact that such species as are present in both faunas are then in nearly 

 the same stage of evolution; and, second, because the diagnostic 

 species of both faunas are less in evidence where the Chemung 

 fauna immediately follows the later stages of the Ithaca fauna than, 

 farther west, where a long interval of time separated the period of 

 the occupation by the Ithaca from the period of the income of the 

 Chemung fauna. 



\To he continued] 



