Shell Cup

The Shell Cup is a cup in the Mario Kart series, introduced in Mario Kart DS as the fifth cup of that game. Typically, it exclusively features classic courses and is the first retro cup overall; these tend to be from the Mushroom Cup and Flower Cup of previous games, and are thus usually simple with few hazards, akin to the Mushroom Cup. The emblem of the cup is a Green Shell.

Mario Tennis Open
The Shell Cup also appears in Mario Tennis Open as the third cup in the Star Open tournament series, but the shell is a Red Shell instead of the usual green. The trophy is designed as a Red Shell on top of a tower that resembles a feature from Bowser's Castle. This cup is held in the Bowser's Castle court, with the matches having three sets for the starting round, three sets for the semifinals, and five sets for the finals.

Completion rewards in the Mario Kart series
Winning the Shell Cup in the Mario Kart series can award the player with various bonuses, depending on the game. Additional cups unlocked are excluded from this list.

Mario Kart DS

 * All retro cups (includes Shell Cup) on 50cc: Daisy
 * All retro cups (includes Shell Cup) on 100cc: Waluigi
 * All retro cups (includes Shell Cup) or all nitro cups (excludes Shell Cup) on 150cc: Seven karts for each playable character
 * All cups on 150cc: 150cc Mirror class
 * All retro cups (includes Shell Cup) or all nitro cups (excludes Shell Cup) on 150cc Mirror: R.O.B.
 * All cups on 150cc Mirror: All karts for each playable character

Mario Kart Wii

 * 1 star rank or higher on all retro cups (includes Shell Cup) on 50cc: Cheep Charger
 * 1 star rank or higher on all retro cups (includes Shell Cup) on 100cc: Bowser Jr.
 * 1 star rank or higher on all retro cups (includes Shell Cup) on 150cc: Jetsetter
 * All cups on 150cc: 150cc Mirror class
 * 1 star rank or higher on all cups on 150cc Mirror: Rosalina

Mario Kart 7

 * All cups on a single cc class: Mii
 * 150cc Shell Cup: Shy Guy
 * All cups on 50cc, 100cc, and 150cc: 150cc Mirror class

Mario Kart 8
Most unlockable characters in Mario Kart 8 are unlocked at random upon winning Grand Prix cups (separate cups must be played to unlock characters), which includes the Shell Cup. Characters that can be unlocked include Rosalina, Metal Mario, Lakitu, Toadette, Baby Rosalina, Pink Gold Peach, Iggy Koopa, Roy Koopa, Lemmy Koopa, Larry Koopa, Wendy O. Koopa, Ludwig von Koopa, Morton Koopa Jr., and Mii. Mii is unlocked upon the eighth Grand Prix cup victory, unless an amiibo is used to unlock Mii beforehand.

In addition, the Gold Standard body can be unlocked by getting a 1 star rank or higher on all non-DLC cups (including the Shell Cup) on any one engine class, and the 150cc Mirror class can be unlocked by winning all 150cc cups.

Mario Kart 8 Deluxe

 * 1 star rank or higher on all cups on 150cc Mirror: Gold Standard
 * All cups on 200cc: Gold Mario

Trading Card info
"The first of four Retro Cups, the Shell Cup is made up of four great blasts from the past. One track from GameCube, one from DS, one from Super NES, and one from N64. It's a great mix, and they all get a face lift with the graphics boost in Mario Kart Wii."

Trivia

 * Mario Kart 7's Shell Cup is the only Shell Cup without a returning Mario Circuit, which is instead part of the Banana Cup.
 * None of the races from Mario Kart 8 (Deluxe)'s Shell Cup have appeared in Mario Kart Tour.

Trofeo Guscio Panzer-Cup