Triassic of Polaad. In R.W.M. van Soest, Th.M.G. van Kempen, and J.C. Braekman

As in many other invertebrate groups, tl-e Mesozoic hexactinellid sponges, especially the abundant and relatively well known Jurassic and Cretaceous ones, differ profoundly from those of the Paleozoic. The Paleozoic hexactinellids represent nearly the entire lyssacinosan grade of evolution i.e. loose, or only partly fused choanosomal skeletons. They may also contain spicules with some rays of the basic hexactine suppressed, up to diactine stage. To the contrary, the Jurassic and Cretaceous faunas are dominated by hexactinellid sponges of the order Hexactinosa, in which a choanosomal skeleton is composed exclusively of hexactines fused into a more or less regular linear series called dictyonal strands. The Triassic sponge faunas are thus crucial to understanding this evolutionary discrepancy. Most unfortunately hexactinellid sponges of this age, as often is the case with other coeval groups of marine organisms, remain poorly known. The reviewed papers dealing with new Triassic faunas could potentialy be very important for students of fossil sponges. They may be treated here jointly not only because of common author and time of publication, but also by being based on the same material with even the same photographs, and text-figures