Magma gets its name (sex undetermined) apparently by being often sighted with Crater. Since magma feeds the lava that erupts from a crater, the two are linked. I've seen Magma each year since 2011 but never with Crater.
Magma has fairly narrow flukes with lots of bright white on the ends and these are my best clues for identification. When I confirm with my photos, I look for the obvious black line near the outer end of the white on its right fluke and the two diffuse black spots next to a black smudge that almost looks like a happy face on the left fluke. Just about every photo I have of Crater shows quite the population of barnacles on both fluke trailers.