How to make a simple dichotomous key for taxonomic identification

Dichotomous (meaning, to cut into 2 pieces)

Dichotomous keys are based on the use of pairs of contrasting statements.  That is, the pairs of statements are designed so that if a characteristic isn't described by one statement, it must be included in the contrasting statement.  By setting up pairs of contrasting statements, you can set up a working key that leads the user to the correct identification.  Dichotomous keys occur throughout Feldmann & Hackathorn's Fossils of Ohio, although they are designed for a wider range of time periods and genera than your own.  However, they can still serve as excellent models for your own keys. 

An Example of a Key designed for the Waynesville/Liberty/Whitewater Formations:

Suppose you were assigned Ostracodes (microscopic crustaceans) as your taxonomic group.  Here is a dichotomous key for 4 genera found in the Waynesville/Liberty/Whitewater Formations:  Ceratopsis, Drepanella, Milleratia, and Tetradella.  The pairs of contrasting statements are colored here-there are 4 pairs that can do the job for these four ostracode genera from the fieldsite.

1A  Shells are smaller than 4 mm long, and have growth lines................................................see Pelecypods or Brachiopods

1B  Shells are smaller than 4 mm long, and have no growth lines............................................2

2A Shell surface has many pronounced ridges and deep grooves ..........................................3

2B Shell surface is smooth, bean-like, with 1 deep central groove (sulcus) ........................Milleratia

3A  Shell shape is subrectangular to oval, with 2 or more grooves arranged vertically ...........................4

3B  Shell shape is sub-trapezoidal, with vertical and horizontal grooves ................................Drepanella

4A  Shell has 2 vertical, curved grooves ............................................................................Ceratopsis

4B  Shell has 4 to 5 vertical, curved groooves.................................................................Tetradella