florideæ

from The Century Dictionary.

• noun An order of (chiefly) marine algæ of a red or purple color.

from the GNU version of the Collaborative International Dictionary of English.

• noun plural (Bot.) A subclass of algæ including all the red or purplish seaweeds; the Rhodospermeæ of many authors; -- so called from the rosy or florid color of most of the species.