Selaginella arenicola subsp. riddellii
Ann. Missouri Bot. Gard. 42: 24. 1955.
Leaves (aerial stems) usually loosely appressed; base often glabrescent. Strobili sometimes with apical vegetative growth, not distinctly larger than subtending stem. Sporophylls usually not abruptly tapering toward apex; apex straight.
Habitat: Mostly on granite outcrops, granite boulders, or gravelly or sandy soil
Elevation: 0–200 m
Ala., Ark., Ga., La., Okla., Tex.
The specimens of Selaginella arenicola subsp. riddellii from Texas are more constant in their morphologic characteristics and thus more easily separated from subsp. arenicola.
"thick" is not a number.