UCMP MYSTERY FOSSIL #31 is . . .
Rhizostomites

Rhizostomites is a scyphozoan, a true jellyfish, and belongs to a subgroup that lacks tentacles. It is about 45 cm across though some modern relatives achieve a diameter twice that. We saw a box jelly fossil (a cubozoan) way back in Mystery Fossil #1. Find out more about scyphozoans here.