Ostracods (or ostracodes) are a class of mostly tiny bivalved crustaceans that are common in most aquatic environments and there are a few terrestrial species. They swim using two large antennae with long setae.
Their shells are composed of magnesium calcite and are usually smooth but some can be complexly sculptured. They are usually ovate to kidney-shaped and have a simple hinge line that might consist of crenulations or a ridge or groove.
They have a small cluster of adductor muscles attached to the middle part of the valves.
For more information on these tiny bivalved crustaceans see:
https://keys.lucidcentral.org/keys/v3/TFI/start%20key/key/crustacea%20key/Media/HTML/Ostracoda.html
