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Using which factors was this id made and how can the species can be distinguished from the Early Dog-violet and Sweet Violet?
I initially used a Dorling Kindersley pocket guide to ID it, but your question got me started wondering.
I still think it's V. riviniana because the leaves look ovate, the veins on the lower petal are quite widely spreading, the spur is lighter than the petals, and the sepals appear long and slender.
Here is a link by the Botanical society of the British Isles, which might be useful: http://www.bsbi.org.uk/Viola.pdf
I'm quite new to all this, so possibly some of the more experienced members will be able to give you better guidance than this.
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