DEMOSPONGES AND HEXACTINELLID SPONGES FROM THE LOWER DEVONIAN ROSS FORMATION OF WEST-CENTRAL TENNESSEE

A fauna of eight taxa of demosponges and hexactinellid sponges has bee n collected from the Lower Devonian (Lochkovian) Ross Formation, large ly out of the upper Birdsong Shale Member in Benton, Decatur, and Ferr y Counties in west-central Tennessee. The Upper Birdsong Shale (''bryo zoan zone'') in which the sponges are most common appears to have been deposited below normal wave base in a quiet marine environment, and r epresents a terrigenous elastic sediment influx onto a carbonate shelf that had existed in the area from at least the middle Silurian. Bento n Quarry in Benton County was the most productive locality for fossil sponges. The new demosponge genera and species Ginkgospongia foliata a nd Coniculospongia radiata occur with the new species Haplistion lobat um and skeletal mats of fine spicules, along with moderately rare spec imens of Hindia sphaeroidalis Duncan. The new hexactinellid genus and species Stiodermiella amanita and Stiodermiella tetragona are characte rized by peculiar ornamented papillose, swollen spicules that produce a massive, armored layer on the upper part of the sponge. The latter a re associated with the new hexactinellid species Twenhofelella bulbulu s, which has relatively normal-appearing hexactines, and with an indet erminate hexactinellid genus, which has spinose hexactines in irregula r orientation in a small, platelike fragment. Root tufts of probable h exactine origin also occur. Swollen spicules in Stiodermiella are remi niscent of swollen spicules in the family Stiodermatidae Finks, largel y from the Permian of western Texas, but elements of the family are al so known from Lower Carboniferous to Permian rocks in Europe and North America.