Vol. XI] DICKERSON— FAUNA OF THE VIGO GROUP 21 



Importance of Guide Fossils 



Good guide fossils are far more difficult to select in connec- 

 tion with tropical Tertiary faunas of the Philippines than in 

 the Californian Tertiary owing to the great predominance of 

 recent mollusca. As will be seen from a study of the fauna 

 cited above, most of the forms which are extinct were originally 

 described from a correlative horizon in Java. Of these, the 

 writer is inclined to think that Cerithium jenkinsi, C. hcrklotsi, 

 C. bandongcnsis; Mitra javana, M. jenkinsi, M. junghuhni, M. 

 bucciniformis; Turris coronifcr; Terebra bicincta, T. javana; 

 Vicarya callosa; Vermetus javanas will probably prove reliable 

 guides among the mollusca. These species are all representa- 

 tives of highly organized genera and their extinction during the 

 post-Miocene time was probably due to their inability to obtain 

 life conditions suited to their highly specialized needs. 



Corals, echinoderms, and the more highly organized foram- 

 inifera will probably prove to be even better horizon determin- 

 ers, but their comparative infrequence in strata of the Philip- 

 pines will at times preclude their use. The writer has not yet 

 attempted to identify the corals and the echinoderms in the 

 collections made, but their value will no doubt prove to be 

 great. It seems that their rate of evolution may have been 

 greatly retarded, but much study will be required in this con- 

 nection. For stratigraphic work in the tropics large and com- 

 plete collections are necessary to obtain results of any value, 

 as the geologic and paleontologic history is read, even with the 

 best data available, with much difficulty. Much comparative 

 material, both recent and fossil, should be accumulated as sub- 

 specific differences will be recognized only through comparative 

 studies. These subspecific differences are exceedingly import- 

 ant for minute separation and discrimination of strata deposited 

 under tropical conditions. 



Factors Promoting Evolution of Pelecypods and 

 Gastropods 



The changes in conditions of environment of marine pelecy- 

 pods and gastropods — in salinity, temperature, depth of water, 

 character of the bottom, food, oceanic currents — determine the 



