仕 切 線
The shikirisen are two flat, enamelled pieces of wood or metal (I think it must be wood, but I don't know this for sure) set in the clay of a sumo ring. Located in the middle of the fighting area, 70 cm apart, respectively east and west of the center they are, in fact, the "starting lines". But that should be taken with a grain of the salt of inaccuracy: the wrestlers are supposed to line up behind their shikirisen, but there are no rules concerning how close to - or far from - them they have to be.
The lines are white, 6*90 cm, parallel, and level with the floor of the dohyo. They easily get covered by sand and dirt during a match, and are meticulously swept clean by the yobidashi between bouts.
During the yokozuna dohyoiri (ring opening ceremony) the yokozuna will place their feet on the lines; not because they have to, but because the shikirisen nicely mark the center of the dohyo, which is where they need to be.
My sources are Sumoforum.net and countless hours of watching sumo.