Two inlet Ports feeding each Inlet valve.

Tumble & Swirl
These can be used to control the amount of tumble and swirl at different RPM's, by balancing the flow from each port using independent butterfly's or barrel valves significant improvements in torque and horsepower have been found. The hole in the middle of each port is where the valve guide lives. Generally, engines that have high port/valve stem angles give high torque where lower angles give much better power with a trade off in torque, (a straighter port will fill better at high rpm). Modern superbikes have low port/valve angles, carbs are now under the tank on all modern high performance engines, and torque is improved by other techniques or considered of less importance. Eg: Yamaha R6.