Chop the shrimps in half and place in a bowl. Marinate the shrimps by pouring the lime juice in the bowl until they are completely covered and are floating freely. Cover the bowl and place in the fridge for 4 hours for best result (or at least 1 hour).
Mix the cilantro, chopped jalapeno (seed removed),chopped tomato, and scallions in a separate large bowl. Add salt and pepper to taste. Completely drain the shrimps using a colander and add to the large bowl. Gently toss until all the ingredients combine. Add the chopped avocado and squeeze some lime over the mixture.
Prepare lettuce leaves on a plate and transfer one or two tablespoons of ceviche into the leaves. Serve. As an alternative, serve the ceviche in a dish with the lettuce leaves all around so that the guests can serve themselves.