Large samples of the levels 35,39 and 41 of the Boom Clay Formation (Rupelian, Oligocene, Belgium) were examined. Preliminary prospections (STEURBAUT & HERMAN, 1979) allowed to consider these levels as the richest ones for both teleost and chondrichthyan remains.Despite the bulk of sediment residues sorted, faunistical enrichment is uncredibly poor. The examination of the odontology of recent relative taxa (HERMAN, HOVESTADT-EULER & HOVESTADT, 1990) allows to consider the subgenus Bythaelurus as a genus of the subfamily Pentanchinae. Scyliorhinus aff. coupatezi is attributed to the genus Bythaelurus , as Bythaelurus steurbauti nov. sp.Oral and rostral teeth of Pristiophorus rupeliensis are redescribed and reillustrated.The taxa of the Rajoidei formerly described (1979) were reexamined: Raja terhaegeniensis and Raja heinzelini are synonymized with Raja cecilae . The presence of a Dasyatididae is signalled.The extraordinary frequency (circa 70% of the fauna) of Squalus alsaticus is confirmed and the discovery of a little curculionid insect is mentioned.