Calamites is a name (= form-genus) for compression/impression and pith casts of arborescent sphenophytes related to modern horsetails. Calamites can be distinguished from a separate form-genus called Archaeocalamites, but technical issues of naming as well as imperfect preservation of fossils makes separating these genera in the field problematic. For convenience, we will use Calamites for all stem fossils as well as for the entire plant.
Some Calamites grew to over 60 feet (20 meters) tall and bore leaves in clusters that, found separately, are placed in the form-genera Annularia and Asterophyllites. Calamites and their leaves are commonly found in Carbonferous rocks in Oklahoma.
Calamites may be found as cylindrical casts or elongate compressions/impressions. In either form, Calamites may be recognized by grooves/lines running the length of the specimen that are regularly and closely spaced and horizontal grooves or "nodes" that are widely spaced and might not be regularly spaced.
In some cylindrical casts, small circular bumps appear along the horizontal nodes. If the small bumps are present, your specimen is a cast (sediment infilling) of the pith—the central portion of Calamites inside the wood of the stem. If bumps are not present, your specimen is most likely a cast of the inner surface of the bark outside of the wood of the stem.