What about a hexactinellid sponge like Hydnoceras? Can't make out the
texture well enough to tell for sure, but it looks like it could be and
the regularly spaced protuberances suggest that to me.
--
George F. Engelmann
Dept. of Geography and Geology
University of Nebraska at Omaha
Omaha, NE 68182
(402) 554 4804
engelman@unomaha.edu