It comprises two identical semi-circular castings which are bolted together to form the wheel.
It is driven from above by the vertical shaft.
Each of the four original millstones was individually driven by a vertical shaft called the quant, from the great spur. Note the single example here.
The gear wheel at the top of the quant is called the stone nut. There was a mechanism to engage or disengage each stone nut depending on which millstones were to be driven at any time.
Note that there are teeth on the lower edge of the great spur wheel. These were used to drive a cog wheel on a horizontal shaft, which in turn provided power to the floor below via a pulley wheel and a flat leather belt.