Preheat the oven to 350°F. Set out a 9 x 13 inch baking dish, or a large rimmed baking sheet.
Gently remove the stems from the mushroom caps, leaving clean cavities. Dust off any dirt or debris with a damp paper towel and place the mushroom caps in the baking dish.
Set out a large mixing bowl. Combine the softened cream cheese, drained crab meat, chopped clams, tiny shrimp, chopped scallions, minced garlic, lemon juice, celery salt, and Cayenne pepper. Mix well until smooth.
Use a spoon to fill the top of each mushroom cavity with a slight dome of seafood stuffing.
Crush the Ritz crackers and place them in a small bowl. One at a time, roll the top of each seafood stuffed mushroom in the Ritz crackers to create a crispy coating over the stuffing. Set the mushrooms back in the baking dish.
Bake for 35 to 40 minutes, until the mushrooms are cooked through, and stuffing tops are golden-brown.
Garnish with additional chopped scallions if desired. Serve warm.
