Key to the species of Cycas:
- CYC1: Ovules coated with hair.
- CYC2: Ovules lacking hair.
- CYC3: Expanded portion of Megasporophyll leaf with closely parallel toothlike projections.
- CYC4: Expanded portion of megasporophyll leaf without closely parallel toothlike projections.
- CYC5: Male cones are stiff and rigid
- CYC6: Male cones are soft
- CYC10: Spongy tissue present in seeds.
- CYC11: Spongy tissue not present in seeds.
- CYC8: Soft seed coat is not fibrous; hard seed coat is studded with wart-like projections
- CYC9: Soft seed coat is fibrous; hard seed coat is smooth.
- Cycas curranii: Pinnae over 6 millimeters wide; seeds over 40 millimeters long.
- Cycas wadei: Pinnae less than 6 millimeters wide; seeds less than 40 millimeters long.
- CYC14: Top of megasporophyll spine much longer and wider than lateral spines
- CYC15: Top of megasporophyll spine similar to lateral spines
- Cycas lindstromii: Male cones less than 5 centimeters in diameter; Stems underground
- CYC17: Male cones over 5 centimeters in diameter; stem rises at least partially above ground.
- CYC20: Seeds have a wing on the tip
- CYC21: Seeds do not have an obvious wing on the tip
- CYC12: Hard seed coat has a fibrous layer
- CYC13: Hard seed coat does not have a fibrous layer.
- CYC37: Basal pinnae gradually reducing in size
- CYC38: Basal pinnae not gradually reducing in size
- CYC49: Pinnae less than 11 millimeters wide.
- CYC50: Pinnae over 11 millimeters wide
- CYC54: Pinnae simple
- CYC55: Pinnae dichotomously divided
- Cycas chamaoensis: Male cones rounded and tapering from the middle toward each end; Spindle-shaped
- CYC63: Male cones elliptical or oval-shaped
- Cycas hongheensis: Leaves have a distinct longitudinal ridge and covered with hair.
- CYC19: Leaves mostly flat and at least the top is without hair.
- Cycas rumphii: Megasporophyll lateral spines are small bumps
- CYC22: Megasporophyll lateral spines distinct
- CYC23: Pinnae are broad, very glossy, and bright green
- CYC24: Pinnae are slightly glossy; Pinnae are bluish when new
- Cycas micronesica: Apical spine is less than 15 millimeters long
- CYC25: Apical spine is over 15 millimeters long
- Cycas thouarsii: Fewer broad, short cataphylls present. Plants of Madagascar
- Cycas zeylanica: Numerous thin, long cataphylls present. Plants of Sri Lanka.
- CYC28: Pinnae less than ten millimeters wide
- CYC29: Pinnae over ten millimeters wide.
- CYC30: Lateral megasporophyll teeth less than four millimeters long.
- CYC31: Lateral megasporophyll teeth over four millimeters long.
- Cycas circinalis: Megasporophyll lamina 75 to 100 millimeters long. Plants of India
- Cycas nathorstii: Megasporophyll lamina 40 to 70 millimeters long. Plants of Sri Lanka
- Cycas chamberlainii: Lateral spines on megasporophyll lamina over ten millimeters long.
- CYC33: Lateral spins on megasporophyll lamina less than 10 millimeters long.
- Cycas macrocarpa: Lateral teeth of megasporophyll over seven millimeters long.
- CYC34: Lateral teeth of megasporophyll less than seven millimeters long.
- Cycas apoa: Lamina lacking lateral spines
- CYC35: Lamina has distinct lateral spines or teeth.
- Cycas inermis: Stems 1 to 4 meters tall; 8 to 14 centimeters in diameter; Petiole is 60 to 80 centimeters long.
- Cycas scratchleyana: Stems over 4 meters tall; 12 to 20 centimeters in diameter; Petiole is 20 to 70 centimeters long.
- CYC39: Lateral megasporophyll teeth large; Basal pinnae reducing to spines.
- CYC40: Lateral megasporophyll teeth small; Basal pinnae not reducing to spines.
- CYC41: Hypodermis present; Leaf edges recurved.
- CYC42: Hypodermis absent; Leaf edges flat.
- CYC68: Pinnae edges flat or recurved; Pinnae more than five millimeters wide.
- CYC69: Pinnae rolled backward or downward; Pinnae less than five millimeters wide.
- CYC43: Hypodermis continuous; Leaves grey or blue.
- CYC44: Hypodermis interrupted; Leaves green.
- CYC45: Leaves strongly glaucous; Male cones oval-shaped.
- CYC46: Leaves not glaucous, or only slightly; Male cones large and spherical.
- CYC47: Spines and/or bristles present on cataphylls
- CYC48: Spines and/or bristles not present on cataphylls
- CYC76: Pinnae edges rolled backward or downward; Pinnae less than four millimeters wide.
- CYC77: Pinnae edges flat or recurved, but not rolled backward or downward; Pinnae over four millimeters wide.
- Cycas megacarpa: Leaves are keeled over 135 degrees.
- CYC75: Leaves are flat, or at least keeled less than 135 degrees.
- CYC79: Cataphylls and crown densely orange and wooly.
- CYC80: Cataphylls and crown gray or white.
- CYC52: Plants relatively large
- CYC53: Plants relatively small
- CYC56: Microsporophylls coming to a short, abrupt point.
- CYC57: Microsporophylls rounded
- CYC58: Pinnae dichotomously divided two to three times.
- CYC59: Pinnae pinnately divided and dichotomously divided more than three times.
- Cycas micholitzii: Leaves 110 to 240 centimeters long and moderately keeled (inserted at 90-120 degrees on rachis)
- Cycas multifrondis: Leaves 200 to 500 centimeters long and flat to slightly keeled (inserted at 120-180 degrees on rachis)
- CYC61: Male cones are small and narrow
- CYC62: Male cones are large
- Cycas pectinata: Has long apical microsporophyll spine.
- CYC65: Has a short apical microsporophyll spine.
- Cycas furfuracea: Pinnae margins flat or recurved; pinnae are over five millimeters wide
- Cycas pruinosa: Pinnae margins are rolled backward or downward; pinnae are less than five millimeters wide.
- Cycas arenicola: Pinnae four to seven millimeters wide; 180-270 pinnae present.
- Cycas calcicola: Pinnae two to four millimeters wide; 200-400 pinnae present.
- Cycas conferta: Pinnae the same color throughout and have stomata on upper and lower surfaces.
- CYC71: Pinnae more or less different colors throughout and only have stomata on one side.
- CYC72: Cataphylls pale brown and hairy
- CYC73: Cataphylls orange and wooly
- Cycas orientis: Midrib more prominent below than above.
- CYC74: Midrib equally prominent above and below.
- Cycas media: Pinnae are less than nine millimeters wide; Cataphylls are persistent.
- Cycas silvestris: Pinnae are over nine millimeters wide; Cataphylls are deciduous.
- Cycas couttsiana: Main growth mostly gray or white trichomes.
- CYC78: Main growth mostly orange or brown trichomes.
- Cycas ophiolitica: Pinnae six to eight millimeters wide; Leaves keeled at 80-120 degrees.
- Cycas platyphylla: Pinnae four to six millimeters wide; Leaves keeled at 45-60 degrees.
- Cycas campestris: Lateral spines of megasporophyll lamina over 8 millimeters long.
- CYC81: Lateral spines of megasporophyll lamina less than 8 millimeters long
- Cycas semota: Megasporophylls 18 to 25 centimeters long; Apex 15 to 35 millimeters wide and not dilated.
- Cycas tuckeri: Megasporophylls 10 to 15 centimeters long; Apex 35 to 50 millimeters wide and dilated.