Shell bits are one of the oldest types of bit you will find, they have even been found in Viking era graves. The ones above probably date from the early 1800s as they are designed to fit a hand brace with a tapered square chuck. Shell bits and their more substantial big brother, the spoon bit, cut with a sharp rounded tip. These have a hollow center section which is thicker on one side, rather than being a constant thickness. Unlike auger bits they do not remove the waste as you cut. They are very quick to work with and you can change the direction of the hole in mid cut, which makes these popular in chair making. The hole they cut is not perfectly circular. Instead the hole is almost square in two of the corners as the bit cuts through the grain. If you look closely at old furniture that has pegged tenons you may see they used a squarish peg. These bits are idea for this type of joint. There are other variations of this bit like the one below. This has a folded cutting edge at the tip. In practice they work in the same manner as a shell bit.