| 1a. | Pectoral fins in two separated lobes | Family Liparidae Snailfishes |
| 1b. | Pectoral fins not in two distinct lobes | 2 |
| 2a. | Body eel-like (body length more than 5 times body depth) | 6 |
| 2b. | Body length less than 4 times body depth | 3 |
| 3a. | No caudal fin; body deeply compressed | Family Molidae Molas |
| 3b. | Caudal fin present | 4 |
| 4a. | Dorsal entirely of soft-rays; body limp, skeleton largely cartilaginous | Family Icosteidae Ragfish |
| 4b. | Dorsal fin with spines in anterior portion; body not limp as above | 5 |
| 5a. | Eight to ten dorsal spines; body without scales | Family Cottidae Sculpins |
| 5b. | Four or fewer dorsal spines | Family Stromateidae Butterfishes |
| 6a. | Tip of tail with a distinct caudal fin which may or may not be joined to the dorsal and anal fins | 10 |
| 6b. | Caudal rays not forming a distinct fin, or tip of tail without rays | 7 |
| 7a. | Jaws needlelike, greatly elongated and curved outward | Family Nemichthyidae Snipe Eels |
| 7b. | Jaws not needlelike | 8 |
| 8a. | Anterior teeth pointed, rear teeth molarlike; dorsal fin with spines | Family Anarhichadidae Wolffishes |
| 8b. | Teeth pointed; no spines in dorsal fin; fins covered with heavy skin | 9 |
| 9a. | Caudal fin small and forked; lower jaw protruding | Family Trichiuridae Cutlassfishes |
| 9b. | Caudal fin pointed, confluent with dorsal and anal fins; lower jaw not protruding | Family Zoarcidae Eelpouts |
| 10a. | Body encased in bony plates | Family Syngnathidae Pipefishes |
| 10b. | Body not encased in bony plates | 11 |
| 11a. | Anal fin absent | Family Trachipteridae Ribbonfishes |
| 11b. | Anal fin present | 12 |
| 12a. | Dorsal fin with spines | 14 |
| 12b. | Dorsal fin without spines | 13 |
| 13a. | Caudal fin forked; minute cycloid scales; lower jaw projecting beyond upper | Family Ammodytidae Sand Lances |
| 13b. | Caudal fin rounded and confluent with dorsal and anal fins; no scales; jaws about equal | Family Scytalinidae Graveldiver |
| 14a. | Dorsal fin with low hooklike spines anteriorly and higher soft-rays posteriorly | Family Ptilichthyidae Quillfish |
| 14b. | Dorsal fin entirely of spines | 14 |
| 15a. | Distance from snout to anal origin greater than distance from anal origin to base of caudal fin | Family Pholidae Gunnels |
| 15b. | Distance from snout to anal origin less than distance from anal origin to base of caudal fin | 16 |
| 16a. | Mouth horizontal or oblique, more horizontal than vertical | Family Stichaeidae Pricklebacks |
| 16b. | Mouth nearly vertical | Family Cryptacanthodidae Wrymouths |