Viola labradorica

Note the spur on the bottom petal, which curls up behind the flower. Dog violet can be distinguished from other purple violets by the long spur and by the "beard" of white hairs on the two side petals. Long-spurred violet also has a long spur, but the side petals are hairless, and the flower has dark purple patches. Great-spurred violet has a short, wide spur with a rounded end.