Sharks

1a. no anal fin

    2a. body flattened, raylike, mouth terminal Squatiniformes

    2b. body not raylike, mouth ventral

      3a. snout elongated, sawlike Pristiophoriformes

      3b. snout short, not sawlike Squaliformes

1b. anal fin present

    4a. 6 or seven gill slits, one dorsal fin Exanchiformes

    4.b 5 gill slits, 2 dorsal fins

      5a. no fin spine

        6a. mouth behind front of eyes

          7a. nictitating eyelids, spiral or scroll intestinal valve Carcharhiniformes

          7b. no nictitating eyelids, ring intestinal valve Lamniformes

        6b. mouth well in front of eyes Orectolobiformes

      5b. dorsal fin spines Heterodontiformes