to american Xylaria species in Mycotaxon 26 (1986). It’s based on substrate and spores, but the species are also grouped in macro characters (size and shape).
Other species with the same thick stromata as polymorpha (> 1 cm) in that key are allantoidea, grandis, poitei and schweinitzii. More slender species with similar shape are cubensis, curta and longipes. For a proper ID, microscopy of mature specimen is needed.
I’m sure some more species have been discovered since then too.
To start with, what is the size of these specimen, and what do they grow on?