They look superficially like very small bi valve shellfish, but they are not molluscs or shrimps - they form their own major group, Ostracoda, within the Crustacea. Mostly smaller than a fingernail, their two-part body casings, or carapaces, are familiar components of mud and ooze in today's oceans, lakes and ponds, and also in preserved sedirnenrs from these habitats.