The largest ostracods actually reach a couple of cm, so 5 mm is not unheard of.  They do have calcite shells, but the structure is very different from those of mollusks and similar to the hard parts of certain other crustaceans (e.g., crabs).  They are common in marine and freshwater habitats; a few get into humid terrestrial areas.

Identification of ostracods and other bivalved crustaceans versus mollusks is very easy with the animal present.  However, it can be more tricky with just the shell.  The muscle scars, hinge, and growth pattern are useful indicators, but some Cambrian fossils remain somewhat uncertain in assignment.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487 USA
    [log in to unmask]

That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa