These seedlings are several weeks old and just moved to a new terra-cotta pot. May, 22nd.
I have no idea what may be growing, I planted a bunch of seeds from a ferry~morse mixed cactus seed package. The package potentially has Saguaro, Hedgehog, Fishook Barrel, Dollar Prickly Pear, Desert Prickly Pear, Chrismas Cholla, Cane Cholla, Sanita Rita Pear and Cardo'n seeds.
I'm not a cactus expert and am of the opinion that they are still too small to accurately identify. I planted about ten seeds, and so far only two have germinated but I hear that it is farily common for cactus seeds to take several weeks to months to germinate.