My understanding is that although generic immature green onions are often called scallions, true scallions are Allium fistulosum, aka Welsh Onions.

Thanks nwl_me,
Do you think that the ones sold in the grocery store are true?

Grocery store scallions are typically specific varieties of onions that, although harvested at the immiture green stage of growth, will typically maintain that long slender growth habit for a prolonged time. Seeds for them are available in many seed catalogs nowdays listed as bunching onions.