204 The Philippine Journal of Science 1922 
cent living species; Miocene, 25 per cent; Pliocene, 60 per cent; 
Pleistocene, 90 per cent. It is the writer’s opinion that this 
percentage scale in the Tropics must be considerably widened. 
On this account the careful determination of guide fossils is 
of great economic importance. Good guide fossils are far more 
difficult to select in connection with tropical Tertiary faunas of 
the Philippines than in the California 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 Cerithiwm 
jenkinsi, C. herklotsi, C. bandongensis, Mitra javana, M. jenkinsi, 
M. junghuhni, M. bucciniformis, Turris coronifer, Terebra bt- 
cincta, T. javana, Vicarya callosa, and Vermetus javanus will 
probably prove reliable guides among the Mollusca. These spe- 
cies are all representatives of highly organized genera, and their 
extinction during post-Miocene time was probably due to their 
inability to obtain life conditions suited to their highly special- 
ized needs. 
Corals, echinoderms, and the more highly organized Forami- 
nifera will probably prove to be even better horizon determiners, 
but. their comparative infrequence in strata of the Philippines 
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 re- 
tarded, but much study will be required in this connection. For 
stratigraphic work in the Tropics large and complete collections 
are necessary for obtaining results of much value, in as much as 
the geologic and paleontologic history, even with the best data 
available, is read 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 important 
for minute separation and discrimination of strata deposited 
under tropical conditions. 
Some of the most abundant species and guide fossils for the 
sandstone and shale facies of the Vigo are illustrated in Plates 
2 to 10. 
LEPIDOCYCLINA LIMESTONE FACIES OF THE VIGO GROUP 
The limestones of the Vigo group are characterized by the 
abundance of the large foraminifer Lepidocyclina, associated 
with other Foraminifera. This limestone is in certain places 
