One reason I am confident that this is Prickly Lettuce is that there are obvious prickles on the edges (margin) of the leaves as well as on the midrib of the lower leaf surface. Looking at the pictures to the right, you can see the prickles on the edge of the leaves and can barely make out the prickles on the middle rib going down the backs of the leaves. Prickly lettuce can look a little bit like some Sowthistles (Sonchus L.). They both have prickles on the leaf margin. But one difference is that Sowthistles are not prickly on the midrib of the underside of the leaf.
