def get_releases(listing: list, specie: str) -> list:
    """
    return the list of gencode releases found on the site.
    Highest fist.

    Parameters
    ----------
    listing : list
        list of gencode FTP paths, some ending with "release_nn" or "release_Mnn"

    specie : str
        mouse of human. mouse release numbers are prefixed with "M"

    Returns
    ------
    list
        releases sorted from highest to lowest
    """
    # ignore releases without a consistent system.
    releases = [int(ftpdir[-2:]) for ftpdir in listing if ftpdir[-2:].isdigit()]
    releases = sorted([str(n) for n in releases if n > 21], reverse=True)
    if specie == "mouse":
        releases = ["M" + r for r in releases]  # dear gencode: why?
    return releases