What is 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 called?

florideae — 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. (Webster's 1913.)