Well considering you had them outside in the intense sunlight with a clear plastic dome in 90-ish degree heat, I'd say you cooked them. Best to reseed and try germinating them in a window sill before acclimating them to outdoors.

Rodney

Start lettuce seeds indoors with 74 degree heat or less. Soak the planting medium fully. Plant seeds on top. Cover the seeds with a some medium and fully soak. L:eave dome off. When the seeds germinate, take them outdoors where they get pretty good lighting and don't leave them out in rainstorms...works perfectly for me.

I start mine in plug trays indoors and immediately put under the lights. They will germinate in about 3 days. I leave them under the lights for about 2 weeks and then harden off outside for a week or so. I just planted out my first batch of Little gem baby romaine transplants last night.

i start lettuce in the shade. photodormancy must not be important to lettuce.