They are Citrus maxima. They have them in stores around here, ocasionally. They're usually a couple of dollars each. To me, they're not worth it. You start out with a fruit that's twice as big as a grapefruit but, because of the thick pith, the edible portion is only as big as a regular grapefruit.
Citron is Citrus medica, The rind is used, candied, in baking. The little green cubes in a fruitcake are citron. Citrons are usually oval shaped, but I had one variety called Buddah's Hand, pictured below. ( I saw one of these, about 6 inches long, in the supermarket, the other day. They wanted $7.00 for it.)
