From the Super Mario Wiki
The Shell Cup is a representation of four cups in Mario Kart DS, Mario Kart Wii and Mario Kart 7, each of which is taken from the previous Mario Kart games. Shell Cup's symbol is a Green Shell and it features tracks from previous Mushroom Cups and Flower Cups. The tracks are usually quick and simple with little hazards. It is the first of the four Retro Cups. The cup's length and difficulty is on par with the Mushroom Cup.
Mario Tennis Open
The Shell Cup also appears in Mario Tennis Open as the third cup in the Star Open tournament series. However, the shell is a Red Shell instead of the usual Green Shell. The design for the cup is a Red Shell on top of a tower that resembles a feature from Bowser's Castle. The court that players participate in this cup is Bowser's Castle.
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