Humans like to organize things. I suspect the author of the lettuce section in Flora of North America is like that. An alleged bunch of organized distinctive species are keyed and described there. Good luck with that! Michigan Flora better approaches reality, discussing some of the bewildering array of characters in this genus. Most of the species can have white, yellow or blue flowers. The inflorescence can be compact, like the yellow one shown on the wildflower page, or diffuse like here. The leaves can vary from no teeth to deeply lobed, often on the same plant. Messiest of all, they hybridize with each other. The one shown here is most likely a cross between woodland (L. floridana) and tall blue. In theory, you can tell them apart by the number of flowers in a head, and the color of the fuzz on the seeds (white or tan). In this population, flower numbers were mostly intermediate, and some had white fuzz, others tan. So when you see a lettuce, admire the flowers and move on. Trying to be more specific can be tricky as trying to eat it.