No zonal fossil has been recognized for the British Purbeckian strata, but the horizon is approximately equivalent to that of "Pensphinctes transilorius" of the European continent. The Purbeckian equivalents of Spilsby and Speeton are in the zone of "Belemnites lateralis". Other marine fossils are "Hemicidaris purbeckensis" and "Ostrea distonta", the latter being abundant in the Cinder bed of the Middle Purbeck. The fresh-water mollusca include "Viviparus" ("Paludina"), "Planorbis", "Melanopsis", "Unio", "Cyrena". A large number of insect genera has been found in the Middle and Lower Purbeck Group.