| Xerocassia armata (S. Watson) Britton & Rose, 1930 |
| Xiphocarpus C. Presl, 1830 |
| Xiphocarpus martinicensis C. Presl, 1830 |
| Xylophacos Rydb., 1903 |
| Xylophacos amphioxys (A. Gray) Rydb., 1906 |
| Xylophacos aragalloides Rydb., 1907 |
| Xylophacos melanocalyx Rydb., 1925 |
| Xylophacos tephrodes (A. Gray) Rydb., 1925 |
| Xyphanthus Raf., 1817 |
| Xyphanthus hederifolius Raf., 1817 |
| Zapoteca H.M. Hern., 1987 |
| Zapoteca alinae H.M. Hern., 1989 |
| Zapoteca balsasensis H.M. Hern., 2015 |
| Zapoteca cruzii H.M. Hern., 2015 |
| Zapoteca formosa (Kunth) H.M. Hern., 1987 |
| Zapoteca formosa subsp. formosa (Kunth) H.M. Hern., 1987 |
| Zapoteca formosa subsp. mollicula (M. Martens & Galeotti) H.M. Hern., 1989 |
| Zapoteca formosa subsp. rosei (Wiggins) H.M. Hern., 1989 |
| Zapoteca formosa subsp. schottii (Torr. ex S. Watson) H.M. Hern., 1989 |
| Zapoteca formosa subsp. sinaloana H.M. Hern., 2015 |