AN UNUSUAL UPPER CRETACEOUS (SANTONIAN) HEXACTINELLID SPONGE FROM THEGREAT VALLEY SEQUENCE, WESTERN SACRAMENTO VALLEY, NORTHERN CALIFORNIA

The new farreid hexactinellid sponge Hormathospongia dictyota new genu s and new species, is described from the upper Santonian Dobbins Shale Member of the Forbes Formation of the Upper Cretaceous Great Valley S equence from the west side of the Sacramento Valley, northwest of Sacr amento. The relatively simple skeleton is composed of quadrangularly a rranged hexactines with overlapping rays, an arrangement strikingly si milar to the skeletal structure of early Paleozoic reticulosid hexacti nellids. However, the California Cretaceous sponges clearly show those spicules embedded in siliceous beams that are united to form a solid dictyonal skeletal framework of only a single layer of regular mesh. S uch an occurrence and stratigraphic relationships suggests that the di ctyonine sponges had their origin from the simply spiculed reticulosid hexactinellids rather than from the more complex dictyo-sponges.