I used to be able to purchase seeds for "Spanish Radishes". I can't them in stores any more and I couldn't find them in your on-line catalog. Do you still carry them? They had a black shell which peeled off to reveal a white radish.
The radish you're looking for is an heirloom called "Black Spanish", and comes in both round and long forms and is available from Abundant Life Seed Foundation, P. O. Box 772, Port Townsend, WA 98368. Phone (360) 385-7455.