There are a number of varieties of this lettuce -- some red, some green, some romaine-type, some butter-type. What they all have in common is that they are made up entirely of baby leaves. When you cut the core out, the head falls apart into perfect, salad-sized leaves.
The trade name for these types of lettuce is Salanova. We use Salanova for nice, fresh heads that hold very well in the mid-summer heat, and we also use them in your salad mix!