| 1a. | Jaws absent; mouth forms a toothed sucking disc; one nostril; no paired fins | Family Petromyzontidae Lampreys |
| 1b. | Jaws present; paired nostrils; paired pectoral and/or pelvic fins present | 2 |
| 2a. | One pair of gill openings | 3 |
| 2b. | Five to seven pairs of gill openings, on sides or undersurface | Section B |
| 3a. | One eye on each side of the head | 6 |
| 3b. | Both eyes on the same side of the head | 4 |
| 4a. | Dorsal and anal fins confluent with pointed caudal fin; usually only left pelvic fin developed; pectoral fins absent or reduced to fine membrane | Family Cynoglossidae Tonguefishes |
| 4b. | Dorsal and anal fins not confluent with caudal; caudal not pointed; usually both pelvics developed | 5 |
| 5a. | Pelvic fins asymmetrically placed on sides of body (fin for eyed side on ridge of abdomen, the other not on ridge); eyes and most pigment on left side of body | Family Bothidae Lefteye Flounders |
| 5b. | Pelvic fins symmetrically placed on sides of body; eyes and most pigment usually on right side of body | Family Pleuronectidae Righteye Flounders |
| 6a. | Pelvic fins absent, or reduced to single spine or scale | Section C |
| 6b. | Pelvic fins present (may be modified as cone or disc) | 7 |
| 7a. | Pelvic fins thoracic | 11 |
| 7b. | Pelvic fins abdominal | 8 |
| 8a. | One dorsal fin | Section D |
| 8b. | Two dorsal fins | 9 |
| 9a. | First dorsal fin with rays, the second adipose | Section E |
| 9b. | Both dorsal fins with rays, the first with spines, the second of soft-rays | 10 |
| 10a. | First dorsal fin with a strong, venomous spine at origin; gill covers and bone structures weak and pliable; teeth fused to form grinding plate | Family Chimaeridae Shortnose Chimaeras |
| 10b. | No single stout spine at origin of first dorsal fin; first dorsal entirely of weak spines, the second dorsal of soft-rays; teeth large, caninelike | Family Sphyraenidae Barracudas |
| 11a. | Pelvics thoracic, strongly modified forming a cone or sucking disc on belly | Section F |
| 11b. | Pelvics thoracic and paired, not as a cone or sucking disc, normal (rays and membranes obvious) or barbel-like or clublike | 12 |
| 12a. | Pelvics with more than 5 soft-rays | Section G |
| 12b. | Pelvics with 5 or fewer soft-rays | 13 |
| 13a. | Pelvics with exactly one spine and 5 soft-rays | Section H |
| 13b. | Pelvics with less than 5 rays, the rays either normal or as barbel-like or club-like structures | Section I |