Sponges occur in three large groups. The Hexactinellida, the Demospongiae and the Calcarea. The Hexactinellida are divided into the Hexactinosa, the Lychniscosa and the Lyssacinosa. The first two have a rigid skeleton, whereas the latter are sponges with loose spicules sitting in their soft parts. They are widely spread over the oceans today. It is difficult to study Lyssacinosa, due to their preferred deep sea habitat, and therefore little is known about their way of living. To sutdy fossil Lyssacinosa is even more challenging. Not only are these sponges rare, but the microscleres (the spicules, which are used as tools for identification) are almost always lost in the fossil record. So other tools must be used to describe them. One way is to use the megascleres, spicules which hold the sponge body in shape. This method is used within this work to describe a lyssacinosan sponge fauna from the Upper Cretaceous of Bornholm, Denmark. Three new genera and eighteen new species are described. They are compared with other, Paleozoic Lyssacinosa and with living ones with regard to diversity, geographic and bathymetric distribution, and features such as shape, anchoring modi and wall thicknesses, to discuss adaptation to sedimentation rates, different substrates and water energy. The sedimentology and the faunal composition of the Arnager Limestone (ca. 88 Ma), aside from the sponges, is discussed. Together with the sponge fauna a three-dimensional picture can be drawn to show the conditions and the nature of the habitat the Lyssacinosa lived in.