Viola rostrata

Note the long spur on the bottom petal, which extends behind the flower. Long-spurred violet can be distinguished from other purple violets by the spur, by the lack of any hairs on the two side petals, and by the darker purple spots on the petals. Dog violet also has a long spur, but the side petals are bearded with white hairs, and the flower is more uniformly light purple. Great-spurred violet has a short, wide spur with a rounded end.