This happened to me last year too. I planted a Mesclun mix that was supposed to include arugula, endive, red kale, red and green romaine, salad bowl, and lolla rossa lettuces.

I ended up with mostly arugula and red kale. NOT the 7-mix I wanted :)

I can't quite tell from your picture which lettuce you are growing. I would suggest you Google images of the lettuce names listed on your seed packet and see if any match your plants.

It looks like you have other lettuces in your garden? If so, at least you will have a variety of greens in your salad.

Here is a link that might be useful: GrafixMuse's Garden Spot