Key to adult males of Perigomphus

1 Extreme base of cerci subequal in width to the space between them (Fig. 4); mesal carinate margin of cerci entire, not notched subapically (Figs. 4, 5)......................................................................... basicornis

- Extreme base of cerci notoriously shorter than space between them; mesal carinate margin of cerci notched subapically (Figs. 8, 10)............................................................................................... 2

2 Cerci in dorsal view with the medial angle at extreme base projected toward midline and almost meeting that of opposite in each cercus (Fig. 8); in lateral view (Fig. 9) with a vestigial sub-basal projection............................. angularis

- Cerci in dorsal view (Fig. 10) with the medial margin at extreme base undeveloped and slightly undulate; in lateral view (Fig. 11) with a well-developed sub-basal projection..................................................... pallidistylus