With the botany sharks that circle this site, it is difficult for a quiz to last long.
Scott is correct. This is Selaginella eclipes. Evidently there is still debate as to whether this species is distinct from S. apoda. I am going with S. eclipes based on the lanceolate leaves of the upper stem surface and their long acuminate tips. I have read that the soil acidity is variable, but FNA does say it is from calcareous habitats.
FNA mentions that there is debate as to the validity of this taxon being a distinct species but then says "It is recognized here at the specific level to highlight the problems within this species complex". Wow! A breath of responsible taxonomy. Too bad they are not consistent in this philosophy.