Forty-five discoveries of fossil tentaculites in northwest Malaya form the basis of this report. As in many other areas, ringed and unringed forms of similar dimensions are found together here. Malayan tentaculites are of early to middle Devonian aspect, but on occasion are intimately associated with Ordovician trilobites and with lower Silurian graptolites. It is suggested that this biochronological anomaly can be explained in terms of paleogeography. Malayan tentaculites occur in a miogeosynclinal euxinic facies which appears to owe its origin to the uprise of a confining geanticlinal barrier. So long as the barrier remained submerged, pelagic organisms had access to the surface waters of this euxinic basin. When the geanticline became emergent in Silurian time, however, faunal replenishment was inhibited. Later differential orogenic movements of the geanticlinal crest (Devonian) are thought to have permitted the ingress of a tentaculite plankton and to have disinterred organic remains predating the isolation interval. Hence the tentaculites are now found amongst considerably older fossils. Similar admixture of faunas may occur elsewhere at the corresponding stage of geosynclinal evolution.