| 1a. | Gill openings on sides of body; pectoral fins not attached to side of head | 3 |
| 1b. | Gill openings under body; pectorals attached to sides of head | 2 |
| 2a. | Caudal fin absent; body has rhomboid outline | Family Rajidae Skates |
| 2b. | Caudal fin present; body has rounded outline | Family Torpedinidae Electric Rays |
| 3a. | Eyes on top of head; body flattened | Family Squatinidae Angel Sharks |
| 3b. | Eyes on sides of head | 4 |
| 4a. | Six or seven gill slits | Family Hexanchidae Cow Sharks |
| 4b. | Five gill slits | 5 |
| 5a. | Anal fin absent | 10 |
| 5b. | Anal fin present | 6 |
| 6a. | Upper lobe of caudal fin greatly elongated, about as long as body | Family Alopiidae Thresher Sharks |
| 6b. | Caudal fin shorter than body | 7 |
| 7a. | Origin of 1st dorsal fin over or behind origin of pelvic fins | Family Scyliorhinidae Cat Sharks |
| 7b. | Origin of 1st dorsal fin well ahead of origin of pelvic fins | 8 |
| 8a. | Length of lower lobe of caudal fin more than 1/2 length of upper caudal lobe | Family Lamnidae Mackerel Sharks |
| 8b. | Length of lower lobe of caudal fin less than 1/2 length of upper caudal lobe | 9 |
| 9a. | Gill slits long, extending from well above midbody to middle of throat; teeth small, about 200 in front series of upper jaw | Family Cetorhinidae Basking Shark |
| 9b. | Gill slits not extending onto underside of body; 25-35 teeth in front series of upper jaw | Family Carcharhinidae Requiem Sharks |
| 10a. | Strong spine at the front of each dorsal fin | Family Squalidae Dogfish Sharks |
| 10b. | No spines in dorsal fins | Family Dalatiidae Sleeper Sharks |