The manner in which the ribs terminate at the joint (node) defines three different form genera, which are considered sub-genera by some researchers. Calamites is the most common of the three found in Kentucky. Calamites ribs abut against each other at the joint or node with a subtle sawtooth-like pattern. Ribs in successive intervals between joints are nodes are slightly offset. Straight terminations with no offsets of ribs between nodes define Archaeocalamites. A mix of the two define Mesocalamites (Gillespie and others, 1978). If considered subgenera, the three forms are written as Calamites, Calamites (Mesocalamites) and Calamites (Archaeocalamites). Regardless of which form is used, the joints (nodes) are the key to identification. Also, fossil cordaite leaves (another plant) have similar-appearing ribs and grooves, but can be distinguished from flattened Calamites, because cordaite leaves lack joints (nodes).
Text by Stephen Greb (KGS).