Unbelievable. Possibly a jellyfish from the genus Deepstaria, only described since the 60s, and until know only known from fragments:  

Imperfectly known, many specimens damaged. Two nominal species, information from both combined here based on Russell (1967), Larson (1986), Larson et al. (1988). Bell remarkably thin, broad, delicate … flexing down in “pursing” manner… most specimens reported colorless but deep brown … lining a paler brown recorded once (Larson et al., 1988, as Deepstaria reticulum); more observations needed before value of this as species character can be evaluated. 

Deepstaria enigmatica