The three known species of Trappea from North America all have small, thin walled, ellipsoid to oblong spores.
For descriptions of Trappea, see
Castellano, M.A. (1990). The new genus Trappea (Basidiomycotina, Hysterangiaceae), a segregate from Hysterangium. Mycotaxon 38: 1-9.
States, J.S. (1991). A new false truffle in the genus Trappea (Hysterangiaceae). Mycotaxon 41: 127-133.
For pics of Trappea darkeri, see
http://www.mykoweb.com/CAF/species/Trappea_darkeri.html