803 



however, while other elements are absolutely different. An excellent 

 illustration of" this is afforded by Marglnella glabella L. and Pseudo- 

 marginella v. Maltzan from the isle of Goree, for here gastropod shells 

 which are quite indistinguishable from each other are inhabited by 

 entirely different animals. Lowly developed forms of foraminifera, 

 however, are much more difficult to distinguish than shells of 

 mollusca, so that not only the "species", but also even the genera 

 are often connected with each other by transitions. It is easily 

 understood that for this reason such morphological 

 species may have a large horizontal and vertical 

 distribution, while with the n u m m u 1 i n i d a e, the 

 structure o f w h i c h is so \' e r y complicated, this is 

 by no means the case. The premised longevity of so many 

 foraminifera is probably only the result of our incapacity to dis- 

 tinguish the species on account of their simple shells. 



However this may be, the whole character of the Tertiary fauna 

 of the West-Progo Mountains is in complete agreement with the 

 theory formerly developed by me, according to which the Javanese 

 Sea was separated from the Tethys since the upper Eocene. 



For a determination of the age of the deposits we have in the 

 tirst place the fact that out of 103 determined species of mollusca 

 no more than 7, i.e. 6,8 Vo» survive to the present day. Moreover 

 of the species desci-ibed as new, relatively few related forms could 

 be noted in the recent fauna. In the oldest Neogene sediments until 

 now known in Java, the lower Miocene strata of Rembang, slightly 

 more than 137o recent species are found; hence the deposits of the 

 West-Progo Mountains cannot be younger than these. The question even 

 arises whether they should not perhaps be reckoned to the Oligocene. 



If we investigate the relationship between the fauna of the West- 

 Progo Mountains and the younger and older Tertiary faunas of the 

 Indian Archipelago, it appears that 24 — 25 species from the strata 

 here dealt with are found in the Neogene deposits of the Dutch 

 East Indies, none, on the contrary, in the upper Eocene of 

 Nanggoelan. Further there exist relations with the Gaj-group of 

 Sind (Burdigalian). From what precedes it appears that the West- 

 Progo beds must be placed at the base of the Neogene and cannot 

 belong to the Oligocene. So they are of lower Miocene age. 



For the above reasons the foraminifera caimot serve for a 

 percentage calculation of forms still existing; for the Indian Archi- 

 pelago material for compaj'ison in this respect is entirely lacking. 

 But among this animal group the lower Miocene Miogypsina theci- 

 deaefonnis Rutien occurs in the fauna of the West-Progo Mountains, 



