key culled from Plants of Pennsylvania by Rhoads & Block
1. corola regular or nearly so
2. inflorescences dense, head-like terminal clusters - Pycnanthemum Mountain Mints
2. inflorescences of many-flowered terminal or axillary clusters - Mentha
1. corola conspicuously liped
3. calyx regular or nearly so
4. upper lip of corola hooded
5. calyx lobes prolonged into short, stiff spines - Galeopsis bifida Hemp Nettle
5. calyx lobes not prolonged into spines - Stachys Hedge Nettles
4. upper lip of corola more-or-less flattened - Pycnanthemum
3. calyx distincly 2-lipped; upper calyx lip 3-lobed, lower lip 2-lobed - Clinopodium vulgare Wild Basil
1. infloresnences essentailly terminal, whorls of flowers subtended by bracts or greatly reduced leaves
2, M. aquatica Water Mint (very rare in streambanks SE & SW)
2. inflorescence spike-like, consisting of 3 or more whorls extending down the stem
3. leaves densely pubescent beneath ...M. longifolia (Horse Mint)...M. x rotundifolia (Apple Mint)...M. x villosa (Wooly Mint)
3. leaves glabrous ...M. spicata... ...M. x piperata
2. inflorescences essentially axillary, whorls of flowers subtended by normal or somewhat reduced leaves
4. M. pulegium Pennyroyal
4. leaves with 4 or more lateral veins on each side of the midvein
5. plants generally pubescent
6. calyx 1.5 - 2.5 mm long - M. arvensis Field Mint (Used in Chinese herbal medicine.)
6. calyx 2.5 - 4 mm long - M. x verticillata Mint
5. plants essentially glabrous or with sparse pubescence - M. x gentilis Red Mint
Notes on some eliminations (35 general in original key):
Hedeoma (Mock and American Pennyroyal) sp. are annual.
Blephilia (Wood Mints) mostly West.
Salvia sp. mostly South and with showy flowers.
Cunila origanoides (Common Dittany or Stone Mint) is glabrous and mostly South.
Trichostema dichotomum (Blue Curls) is annual and has showy "blue-curls".
Teucrium canadense (Wild Germander or Wood Sage) has leaves 5-10 cm.
Agastache (Giant Hyssops) sp. are 1 to 1 1/2 meters tall.