Yikes!
Find a pot (with drain holes) a little bigger than the root ball. Pot it in cactus soil with added grit (perlite, pumice, aquarium gravel, non-oyster shell chicken grit...) at a ratio of 2 parts soil, 1 part grit. Dampen the soil (not wet) and plant the cactus roots. Don't water for a week.
Oh, Mammillaria elongata. Its a relatively old plant. Very nice!
