The various fins and other structures mentioned in the keys are named in the accompanying outlines of a haddock and of a typical shark (fig. 1). A simple way to explain the use of the keys is to use the haddock as an example, running it down with the illustration at hand for reference.

Explanation of the use of the keys

Figure 1.—Diagrams of a haddock (below) and of a typical shark (above) with terms used in the keys and descriptions.

Turning to Key A (p. 5), we find that our fish fits the second alternative under section 1, since it has bony jaws and pectoral fins, and is not shaped like an eel. This refers us to section 3.

There being only one gill opening on each side, we go from section 3 to section 5. As our fish does not have a tubular snout section 5 refers us to section 6, and this in turn to section 7, since neither the upper jaw nor the lower is greatly prolonged. Since the body is not square-cut close behind the dorsal and anal fins, but has a definite tail part, we proceed from section 7 to section 8, and from section 8 to section 11, for our fish has no sucking plate or disc, either on top of the head, or on the chest. Section 11 refers us in turn to section 12 because the tail fin is nearly symmetrical in outline. The anal fin being clearly and definitely separated from the caudal fin, we go from section 12 to section 13; and from section 13 to section 14, for our fish does not have any evident light-producing ("luminescent") spots either on its sides or on its head. Our fish does not have a fleshy fin or flap either in front of the ordinary dorsal fins or behind them, but all of its dorsal fins are supported by rays that are visible if held against the light. Consequently, we proceed from section 14 to section 18, end this refers us to section 22, there being no flaps or tags of skin on the sides of the head.[11] Our fish obviously does not lie flat on one side, i. e., it is not one of the flat fishes, which brings us to section 23, and [page 5] this in turn carries us to Key E (p. 7) because it has three separate, well developed dorsal fins.

Since there are 3 dorsal fins and 2 anal fins, section 1 of Key E sends us to the key to the cod and silver hake families (p. 173). Turning to the first section of the latter we find that our fish fits the first alternative (3 dorsal fins and 2 anals), which refers it to section 2. And here the black lateral line and the dark blotch on each shoulder name it a haddock.

Any other Gulf of Maine species is to be named in the same way, starting with Key A, section 1, and following through the appropriate alternatives as they refer it from section to section.

[11] There is a barbel on its chin, but this is very different in appearance from the skin flaps around the jaws that are characteristic of the few species that fall under the first alternative of section 18.