Vol. XI] DICKERSON— FAUNA OF THE VIGO GROUP 25 



ceeded in making a strategic retreat. Even more pronounced 

 were the "accordion"-like changes during the Pliocene, and 

 during the Pleistocene "jig-time" was in vogue. 



As was pointed out above, archipelagic conditions prevailed 

 in the Philippines during the Tertiary, although the record for 

 the Miocene is missing or extremely meager. An archipelago 

 located in the tropics offers a great variety of habitat, and a new 

 species entering such a region could on this account find suitable 

 conditions for existence. 



From Miocene to Recent in the tropics molluscan faunas 

 have changed but little, and but slight specific alterations have 

 occurred as well. Since a tropical or sub-tropical climate pre- 

 vailed over California, Oregon, and Washington during upper 

 Eocene time, the great geographic and stratigraphic ranges 

 of certain species of Tejon (Upper Eocene) age is due to nearly 

 uniform conditions and other factors mentioned. The great 

 stratigraphic range of many Tejon Eocene species is probably 

 due to uniformity in climate during long periods of time and 

 slight faunal changes have greater significance in the Upper 

 Eocene than corresponding changes in the Miocene. Pliocene, 

 and Pleistocene time and these variations probably required a 

 much longer time for their production as well. Uniformity in 

 oceanic temperature enabled many species to range far to the 

 north, and. in fact, far west of California to the Eocene of 

 Japan where Perissolax blakci, Pholadomya nasuta or their 

 near relatives occur 12 . 



The Eocene time then must not be measured by the same 

 faunal "yardstick" as Pliocene and Miocene time, but a much 

 finer scale is required. It is the writer's opinion, based upon 

 such considerations, that Eocene time is far longer than any 

 of the other divisions of the Tertiary. 



Summary 



The tentative conclusion of the writer is that in the study 

 of Tertiary faunas of the tropics, a different percentage scale 

 must be used. For the later Tertiary, Miocene, Pliocene, and 

 Pleistocene the percentages which apply in the temperate re- 



12 Yokoyama, M., Some Tertiary Fossils from the Miike Coal-field. Journal of the 

 College of Science, Imperial University of Tokyo, Vol. XXVII, 1911, Art. 20. 



