Harrison is officially an Army-crawler now. Not a hands-and-knees crawler yet -- that will be next (hopefully soon, for the sake of his clothes). Right now he lays flat on his stomach, props himself up on his elbows, and then pulls himself along / rolls over one of his arms to move forward. His legs contribute nothing to this exercise -- they just get dragged behind.
He can also get up on his hands and knees and rock back and forth, but he hasn't tried reaching forward with a hand yet, so nothing doing on that crawling style!