Sometimes I know the genus but not the species (even to the level of “circa forma”). In this case I use “sp.” to designate “a species in this genus.” Other times I present several species from the same genus. In this case I use the designation “spp.” meaning “several species in the genus.”
Convention allows the abbreviation of a genus to the first letter if the full word has already been used in a paragraph. Thus Erythrina smithiana becomes E. smithiana.