A great amount of fossil ostracodas are well preserved in eastern China。In this paper

the writer wishes to deal mainly with the characteristics of the ostracoda assemblages of the Cretaceous-Tertiary oil-gas bearing seriesand to discuss their geological significances.1.The assemblage charactristics and evolution stages of the ostracodaThere are a great amount of ostracoda in the related series.Eighty percentof about a thousand of the fossils are new species.According to my point ofview

the evolution of ostracodas found from different formations can be subdi-vided into three stages

namely:early and middle Cretaceous

late Cretaceous-early Palaeogene and middle-late Palaeogene.In the early Cretaceous

after theJurasslc volcanism had just terminated

there existed a comparative stablesedimentary environment in eastern China.Except for Fuxin coal basin

in themore coarser clastic sediments of many other regions

only a little amount offossil were found.On the contrary

the middle Cretaceous

with a stablesedimentary Conditions

is widely distributed in Songliao basin

and a greatnumber of ostracoda have been found.The main assemblage is chiefly Cypridea-Ilyocyprimorpha-Limnocypridea.The associated genera and speciesare Timiriasevia

Lycopterocypris

Mongolianella

Harbinia

Kaitunia

Zizipho-cypris.The middle Cretaceous is the major oil-bearing series of the Daq-ing Oil Field.From late Cretaceous to early Palaeogene

the main assemblageof ostracoda is characterized by Cristocypridea-Cypridea-Candona

associa-ted with Candoniella

Cyclocypris

Cyprinotus

Eucypris

Limnocythere

Me-tacypris.In the assemblage

Cristocypridea is clearly marked by the beak atthe front terminal of the shell.It existed even in a short interval of depo-sition

but was widespread and more stable

and so is considered to be astandard genus in the continental facies.The assemblage is usually found inthe corresponding strata in many other basins too

for instance

from theupper Fulongquen formation to Mingshui formation in Songliao basin

Wang-shi group in Bohai Gulf basin

Paomagang formation in Jianghan basin

fromthe upper Fenshuiao formation to the lower Yuanjiang formation in Dongtingbasin

Daijiaping formation in Hengyang basin and etc.It shows that upto late Cretaceous

a more stable sedimentary environment in vast regionsand a transitional assemblage of ostracodas from late Cretaceous to earlyPalaeogene was widely existed.After the last phace of Yenshan move-ment

the area of Songliao basin in Tertiary became much smaller

the sedimen-tary conditions was unstable

and the ostracoda was declined and found a littleonly

such as Candona

Cyprinotus

Candoniella and Ilyocypris.Middle-late Palaeogene formations of great thickness were deposited in Bohai Gulf

Subaiand Jianghan basins respectively.In these isolated basins of different sedi-mentary conditions had been preserved different assemblages of ostracoda called types of Bohai Gulf

Subai and Jianghan correspondently.Type Bohai Gulfconsists of Huabeinia

Camarocypris

Phacocypris

Chinocythere and Dong-yingia.Some of them have a big individual(2mm)and a variety of delicateornaments in there shell surface.They are associated by Candona

Cyprino-tus

Eucypris

Cyclocypris

Limnocythere etc.Type Subai consists of Sinocyp-ris

Ilyocypris

Eucypris

Limnocythere

Cyprisnotus

Candoniella

Caspiolla

Candona etc.Type Jianghan consists of Limnocythere

Eucypris

Cyprinotus

Cypridopsis

Cypris

Ilyocypris

Cyclocypris

Paracandona

Candoniella

Candona

etc.2.The geological significance of the ostracodaThe growth and evolution of the ostracoda are closely related to thetectonic movement and thus reflect the tectonic environment during the depo-sition.For instance

in late Cretaceous

the deposits in eastern China tend-ed to be in comparative stable sedimentary conditions and in more extensiveand flat basins

thus occured the Cristocypridea-Cypridea-Candona assem-blage.But in the Palaeogene

the three different types of assemblages of ostracoda said above

were developed in different basins which were iso-lated each other by the Yangshan movement.Besides

the major oil-ga